Semiconductor device, and display device and electronic device having the same

ABSTRACT

An object is to provide a semiconductor device which can suppress characteristic deterioration in each transistor without destabilizing operation. In a non-selection period, a transistor is turned on at regular intervals, so that a power supply potential is supplied to an output terminal of a shift register circuit. A power supply potential is supplied to the output terminal of the shift register circuit through the transistor. Since the transistor is not always on in a non-selection period, a shift of the threshold voltage of the transistor is suppressed. In addition, a power supply potential is supplied to the output terminal of the shift register circuit through the transistor at regular intervals. Therefore, the shift register circuit can suppress noise which is generated in the output terminal.

This application is a continuation of U.S. application Ser. No.14/143,535 filed on Dec. 30, 2013 (now U.S. Pat. No. 9,406,699 issuedAug. 2, 2016) which is a continuation of U.S. application Ser. No.11/649,876 filed on Jan. 4, 2007 (now U.S. Pat. No. 8,742,811 issuedJun. 3, 2014), which are all incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device. In particular,the invention relates to a shift register which is formed by usingtransistors. In addition, the invention relates to a display devicehaving the semiconductor device and an electronic device having thedisplay device.

BACKGROUND ART

In recent years, since a large display device such as a liquid crystaltelevision is increased, a display device such as a liquid crystaldisplay device or a light-emitting device has been actively developed.In particular, a technique where a pixel circuit and a driver circuitincluding a shift register circuit or the like (hereinafter described asan internal circuit) are formed over the same substrate by usingtransistors which are formed by using an amorphous semiconductor over aninsulator has been actively developed, since the technique greatlycontributes to low power consumption and low cost. The internal circuitformed over the insulator is connected to a controller IC or the likearranged outside the insulator (hereinafter described as an externalcircuit) through a FPC or the like, and its operation is controlled.

In addition, a shift register circuit which is formed by usingtransistors made of an amorphous semiconductor has been devised as theinternal circuit formed over the insulator (see Reference 1: PCTInternational Publication No. 95/31804).

However, since the shift register circuit has a period in which anoutput terminal is in a floating state, noise is easily generated in theoutput terminal. Due to the noise generated in the output terminal, amalfunction of the shift register circuit occurs.

In order to solve the aforementioned problems, a shift register circuitwhere an output terminal does not become a floating state has beendevised. This shift register circuit is operated by a so-called staticdriving (see Reference 2: Japanese Published Patent Application No.2004-78172).

The shift register circuit disclosed in Reference 2 can realize thestatic driving. Therefore, the output terminal does not become afloating state in this shift register circuit so that noise generated inthe output terminal can be reduced.

DISCLOSURE OF INVENTION

In the aforementioned shift register circuit disclosed in Reference 2,its operating periods are divided into a selection period in which oneselection signal is output and a non-selection period in which anon-selection signal is output, and most periods in these operatingperiods become non-selection periods. In the non-selection period, a lowpotential is supplied to the output terminal through a transistor. Thatis, this transistor for supplying the low potential to the outputterminal is on in most periods in the operating periods of the shiftregister circuit.

It is known that characteristics in a transistor which is manufacturedby using an amorphous semiconductor deteriorate in accordance with atime in which the transistor is turned on and a potential applied to thetransistor. In particular, a threshold voltage shift where the thresholdvoltage of a transistor rises becomes obvious when the characteristicsof the transistor deteriorate. This threshold voltage shift is one ofbig causes of the malfunction of the shift register circuit.

In view of the aforementioned problems, it is an object of the inventionto provide a shift register circuit where noise is reduced in anon-selection period and deterioration of a transistor can besuppressed, a semiconductor device or a display device having the shiftregister circuit, or an electronic device having the display device.

In the invention, a transistor included in a semiconductor device is notalways on to suppress characteristic deterioration of the transistor.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, aninverter, a first wiring, a second wiring, and a third wiring. A firstterminal of the first transistor is electrically connected to the firstwiring; a second terminal of the first transistor is electricallyconnected to a second terminal of the second transistor; and a gateterminal of the first transistor is electrically connected to a firstterminal of the inverter. A first terminal of the second transistor iselectrically connected to the second wiring, and a gate terminal of thesecond transistor is electrically connected to a second terminal of thethird transistor. A first terminal of the third transistor iselectrically connected to the third wiring, and a gate terminal of thethird transistor is electrically connected to a second terminal of theinverter. The gate terminal of the first transistor is electricallyconnected to a transistor for making the gate terminal of the firsttransistor into a floating state.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a first wiring, a second wiring,a third wiring, and a fourth wiring. A first terminal of the firsttransistor is electrically connected to the first wiring; a secondterminal of the first transistor is electrically connected to a secondterminal of the second transistor; and a gate terminal of the firsttransistor is electrically connected to a gate terminal of the fourthtransistor. A first terminal of the second transistor is electricallyconnected to the second wiring, and a gate terminal of the secondtransistor is electrically connected to a second terminal of the thirdtransistor. A first terminal of the third transistor is electricallyconnected to the third wiring, and a gate terminal of the thirdtransistor is electrically connected to a second terminal of the fourthtransistor and a second terminal of the fifth transistor. A firstterminal of the fourth transistor is electrically connected to thesecond wiring. A first terminal of the fifth transistor is electricallyconnected to the fourth wiring, and a gate terminal of the fifthtransistor is electrically connected to the fourth wiring. The gateterminal of the first transistor is electrically connected to atransistor for making the gate terminal of the first transistor into afloating state.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a sixth transistor, a firstwiring, a second wiring, a third wiring, a fourth wiring, and a fifthwiring. A first terminal of the first transistor is electricallyconnected to the first wiring; a second terminal of the first transistoris electrically connected to a second terminal of the second transistor;and a gate terminal of the first transistor is electrically connected toa gate terminal of the fourth transistor and a second terminal of thesixth transistor. A first terminal of the second transistor iselectrically connected to the second wiring, and a gate terminal of thesecond transistor is electrically connected to a second terminal of thethird transistor. A first terminal of the third transistor iselectrically connected to the third wiring, and a gate terminal of thethird transistor is electrically connected to a second terminal of thefourth transistor and a second terminal of the fifth transistor. A firstterminal of the fourth transistor is electrically connected to thesecond wiring. A first terminal of the fifth transistor is electricallyconnected to the fourth wiring, and a gate terminal of the fifthtransistor is electrically connected to the fourth wiring. A firstterminal of the sixth transistor is electrically connected to the fourthtransistor, and a gate terminal of the sixth transistor is electricallyconnected to the fifth wiring.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a sixth transistor, a seventhtransistor, a first wiring, a second wiring, a third wiring, a fourthwiring, and a fifth wiring. A first terminal of the first transistor iselectrically connected to the first wiring; a second terminal of thefirst transistor is electrically connected to a second terminal of thesecond transistor; and a gate terminal of the first transistor iselectrically connected to a gate terminal of the fourth transistor, asecond terminal of the sixth transistor, and a second terminal of theseventh transistor. A first terminal of the second transistor iselectrically connected to the second wiring, and a gate terminal of thesecond transistor is electrically connected to a second terminal of thethird transistor and a gate terminal of the seventh transistor. A firstterminal of the third transistor is electrically connected to the thirdwiring, and a gate terminal of the third transistor is electricallyconnected to a second terminal of the fourth transistor and a secondterminal of the fifth transistor. A first terminal of the fourthtransistor is electrically connected to the second wiring. A firstterminal of the fifth transistor is electrically connected to the fourthwiring, and a gate terminal of the fifth transistor is electricallyconnected to the fourth wiring. A first terminal of the sixth transistoris electrically connected to the fourth transistor, and a gate terminalof the sixth transistor is electrically connected to the fifth wiring. Afirst terminal of the seventh transistor is electrically connected tothe second wiring.

A semiconductor device in accordance with one aspect of the inventionincludes a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a sixth transistor, a seventhtransistor, an eighth transistor, a first wiring, a second wiring, athird wiring, a fourth wiring, a fifth wiring, and a sixth wiring. Afirst terminal of the first transistor is electrically connected to thefirst wiring; a second terminal of the first transistor is electricallyconnected to a second terminal of the second transistor; and a gateterminal of the first transistor is electrically connected to a gateterminal of the fourth transistor, a second terminal of the sixthtransistor, a second terminal of the seventh transistor, and a secondterminal of the eighth transistor. A first terminal of the secondtransistor is electrically connected to the second wiring, and a gateterminal of the second transistor is electrically connected to a secondterminal of the third transistor and a gate terminal of the seventhtransistor. A first terminal of the third transistor is electricallyconnected to the third wiring, and a gate terminal of the thirdtransistor is electrically connected to a second terminal of the fourthtransistor and a second terminal of the fifth transistor. A firstterminal of the fourth transistor is electrically connected to thesecond wiring. A first terminal of the fifth transistor is electricallyconnected to the fourth wiring, and a gate terminal of the fifthtransistor is electrically connected to the fourth wiring. A firstterminal of the sixth transistor is electrically connected to the fourthtransistor, and a gate terminal of the sixth transistor is electricallyconnected to the fifth wiring. A first terminal of the seventhtransistor is electrically connected to the second wiring. A firstterminal of the eighth transistor is electrically connected to thesecond wiring, and a gate terminal of the eighth transistor iselectrically connected to the sixth wiring.

In addition, in the invention, the ratio (W/L) of channel width W tochannel length L of the fourth transistor may be equal to or ten timesas large the ratio W/L of channel width W to channel length L of thefifth transistor.

In addition, in the invention, the first transistor and the thirdtransistor may have the same conductivity type.

In addition, in the invention, the first transistor and the fourthtransistor may be n-channel transistors or may be p-channel transistors.

In addition, in the invention, a capacitor which is electricallyconnected between the second terminal and the gate terminal of the firsttransistor may be provided.

In addition, in the invention, capacitance may be formed by using a MOStransistor as a substitute for the capacitor.

In addition, in the invention, the capacitor includes a first electrode,a second electrode, and an insulator which is held between the firstelectrode and the second electrode. The first electrode may be asemiconductor layer; the second electrode may be a gate wiring layer;and the insulator may be a gate insulating film.

In addition, in the invention, a clock signal may be supplied to thefirst wiring and an inverted clock signal which differs in phase fromthe clock signal by 180 degrees may be supplied to the third wiring.

A display device in accordance with one aspect of the invention includesa plurality of pixels and a driver circuit. Each of the plurality ofpixels is controlled by the driver circuit. The driver circuit includesa plurality of transistors and a circuit for not always turning on eachof the plurality of transistors.

In addition, in the invention, the driver circuit may include theabove-described semiconductor device.

In addition, in the invention, each of the plurality of pixels includesat least one transistor. A transistor included in each of the pluralityof pixels and a transistor included in the driver circuit may have thesame conductivity type.

In addition, in the invention, each of the plurality of pixels and thedriver circuit may be formed over the same substrate.

In addition, a display device of the invention may be applied to anelectronic device.

As described above, in the invention, in order not to always turn on thesecond transistor and the seventh transistor, on states or off states ofthe second transistor and the seventh transistor are controlled by thesignal which is supplied to the third wiring.

In addition, in order not to turn on the second transistor when thefirst transistor is turned on, the third transistor is turned off byconnecting the gate terminal of the first transistor to the gateterminal of the second transistor through the inverter. When the secondtransistor is turned off before the third transistor is turned off, thesecond transistor is continuously kept off. Accordingly, the firstwiring and the second wiring are not electrically connected to eachother through the first transistor and the second transistor.

Note that in the case where a potential of the first wiring is changedwhen the first transistor is on and the second transistor is off, apotential of the second terminal of the first transistor is alsochanged. At this time, a potential of the gate terminal of the firsttransistor is changed at the same time by the capacitive coupling of thecapacitor when the gate terminal of the first transistor is in afloating state. Here, when the potential of the gate terminal of thefirst transistor is changed to a value which is greater than or equal tothe sum of the potential of the first wiring and the threshold voltageof the first transistor, or to a value which is less than or equal tothe sum of the potential of the first wiring and the threshold voltageof the first transistor, the first transistor is continuously kept on.In this manner, the invention has a function of turning on the firsttransistor to set the first terminal and the second terminal of thefirst transistor to have the same potentials, even if the potential ofthe first wiring is changed.

Note that a switch described in this specification can employ anelectrical switch, or a mechanical switch, for example. That is, anyelement can be employed as long as it can control a current flow, andthus, a switch is not limited to a certain element. For example, it maybe a transistor, a diode (e.g., a PN junction diode, a PIN diode, aSchottky diode, or a diode-connected transistor), or a logic circuitcombining such elements. Therefore, in the case of employing atransistor as a switch, the polarity (conductivity type) of thetransistor is not particularly limited to a certain type since itoperates just as a switch. However, when off-current is preferred to besmall, a transistor of a polarity with small off-current is preferablyemployed. A transistor provided with an LDD region, a transistor with amulti-gate structure, or the like is given as an example of a transistorwith small off-current. In addition, it is preferable that an n-channeltransistor be employed when a potential of a source terminal of thetransistor which is operated as a switch is closer to alow-potential-side power supply (e.g., Vss, GND, or 0 V), while ap-channel transistor be employed when the potential of the sourceterminal is closer to a high-potential-side power supply (e.g., Vdd).This is because the transistor is easily operated as the switch sincethe absolute value of a voltage between a gate and a source of thetransistor can be increased. Note that a CMOS switch may also beemployed by using both n-channel and p-channel transistors.

Note that in the invention, description “being connected” is synonymouswith description “being electrically connected”. Accordingly, otherelements or switches may be sandwiched between elements.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes and include various elements. Forexample, a display medium, the contrast of which changes by anelectromagnetic action, such as an EL element (e.g., an organic ELelement, an inorganic EL element, or an EL element containing bothorganic and inorganic materials), an electron-emissive element, a liquidcrystal element, electronic ink, or the like can be applied. Note thatdisplay devices using EL elements include an EL display; display devicesusing electron-emissive elements include a field emission display (FED),an SED-type flat panel display (SED: Surface-conduction Electron-emitterDisplay), or the like; display devices using liquid crystal elementsinclude a liquid crystal display; and display devices using electronicink include electronic paper.

Note that in the invention, the type of a transistor which can beapplied is not limited to a certain type. A thin film transistor (TFT)using a non-single crystalline semiconductor film typified by amorphoussilicon or polycrystalline silicon, a transistor formed by using asemiconductor substrate or an SOI substrate, a MOS transistor, ajunction transistor, a bipolar transistor, a transistor using a compoundsemiconductor such as ZnO or a-InGaZnO, a transistor using an organicsemiconductor or a carbon nanotube, or other transistors can be applied.In addition, a type of a substrate over which a transistor is formed isnot limited to a certain type. The transistor can be arranged over asingle crystalline substrate, an SOI substrate, a glass substrate, aplastic substrate, or the like.

Note that as described above, various types of transistors may beemployed in the invention, and such transistors can be formed overvarious types of substrates. Accordingly, all of the circuits may beformed over a glass substrate, a plastic substrate, a single crystallinesubstrate, an SOI substrate, or any other substrates. Alternatively,some of the circuits may be formed over a substrate while the otherparts of the circuits may be formed over another substrate. That is, notall of the circuits are required to be formed over the same substrate.For example, a part of the circuits may be formed by using transistorsover a glass substrate and the other parts of the circuits may be formedover a single crystalline substrate, so that the IC chip is connected tothe glass substrate by COG (Chip On Glass). Alternatively, the IC chipmay be connected to the glass substrate by TAB (Tape Automated Bonding)or a printed circuit board.

The structure of a transistor is not limited to a certain type. Forexample, a multi-gate structure having two or more gate electrodes maybe used. In addition, a structure where gate electrodes are formed aboveand below a channel may be employed. In addition, any of the followingstructures may be employed: a structure where a gate electrode is formedabove a channel; a structure where a gate electrode is formed below achannel; a staggered structure; an inversely staggered structure; and astructure where a channel region is divided into a plurality of regions,and the divided regions are connected in parallel or in series. Further,a channel (or a part of it) may overlap with a source electrode or adrain electrode. Furthermore, an LDD (Lightly Doped Drain) region may beprovided.

It is to be noted that in this specification, one pixel means theminimum unit of an image. Accordingly, in the case of a full colordisplay device which is made of color elements of R (red), G (green),and B (blue), one pixel is formed by using a dot of a color element ofR, a dot of a color element of and a dot of a color element of B.

It is also to be noted that in this specification, when it is describedthat pixels are arranged in matrix, the description includes not only acase where pixels are arranged in a so-called grid pattern by combiningvertical stripes and lateral stripes, but also a case where dots ofthree color elements (e.g., RGB) are arranged in a so-called deltapattern in the case of performing a full color display with three colorelements. In addition, sizes of light-emitting regions may be differentbetween respective dots of color elements.

A transistor is an element including at least three terminals of a gate,a drain, and a source, and has a channel region between a drain regionand a source region. Here, since a source region and a drain region ofthe transistor may change depending on the structure, operatingconditions, and the like of the transistor, it is difficult to definewhich is a source region or a drain region. Therefore, in thisspecification, one of regions functioning as a source region and a drainregion is described as a first terminal and the other region isdescribed as a second terminal.

Note that in this specification, a semiconductor device means a devicehaving a circuit including semiconductor elements (e.g., transistors ordiodes). The semiconductor device may also include all devices that canfunction by utilizing semiconductor characteristics. A display deviceincludes not only a display panel itself where a plurality of pixelsincluding display elements such as liquid crystal elements or ELelements are formed over the same substrate as a peripheral drivercircuit for driving the pixels, but also a display panel attached with aflexible printed circuit (FPC) or a printed wiring board (PWB). Inaddition, a light-emitting device means a device using self-luminousdisplay elements such as EL elements or elements used for an FED.

A semiconductor device of the invention can turn on a transistor, on/offof which is controlled by a signal supplied to a third wiring at regularintervals. Thus, since the transistor of a shift register circuit whichuses the semiconductor device of the invention is not always on in anon-selection period, the threshold voltage shift of the transistor canbe suppressed. In addition, a power supply potential is supplied to anoutput terminal of the shift register circuit which uses thesemiconductor device of the invention through the transistor at regularintervals. Therefore, the shift register circuit which uses thesemiconductor device of the invention can suppress noise which isgenerated in the output terminal.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram showing Embodiment Mode 1;

FIG. 2 is a timing chart showing Embodiment Mode 1;

FIG. 3 is a diagram showing Embodiment Mode 1;

FIG. 4 is a diagram showing Embodiment Mode 1;

FIG. 5 is a diagram showing Embodiment Mode 1;

FIG. 6 is a diagram showing Embodiment Mode 1;

FIG. 7 is a diagram showing Embodiment Mode 1;

FIG. 8 is a diagram showing Embodiment Mode 1;

FIG. 9 is a diagram showing Embodiment Mode 1;

FIG. 10 is a diagram showing Embodiment Mode 1;

FIG. 11 is a diagram showing Embodiment Mode 1;

FIG. 12 is a timing chart showing Embodiment Mode 1;

FIG. 13 is a diagram showing Embodiment Mode 1;

FIG. 14 is a diagram showing Embodiment Mode 1;

FIG. 15 is a diagram showing Embodiment Mode 1;

FIG. 16 is a diagram showing Embodiment Mode 1;

FIG. 17 is a diagram showing Embodiment Mode 2;

FIG. 18 is a timing chart showing Embodiment Mode 2;

FIG. 19 is a timing chart showing Embodiment Mode 2;

FIG. 20 is a diagram showing Embodiment Mode 3;

FIG. 21 is a diagram showing Embodiment Mode 3;

FIG. 22 is a diagram showing Embodiment Mode 3;

FIG. 23 is a diagram showing Embodiment Mode 3;

FIG. 24 is a diagram showing Embodiment Mode 3;

FIG. 25 is a diagram showing Embodiment Mode 3;

FIG. 26 is a diagram showing Embodiment Mode 3;

FIG. 27 is a diagram showing Embodiment Mode 3;

FIG. 28 is a diagram showing Embodiment Mode 3;

FIG. 29 is a diagram showing Embodiment Mode 3;

FIG. 30 is a diagram showing Embodiment Mode 3;

FIG. 31 is a diagram showing Embodiment Mode 3;

FIG. 32 is a diagram showing Embodiment Mode 3;

FIG. 33 is a diagram showing Embodiment Mode 3;

FIG. 34 is a diagram showing Embodiment Mode 3;

FIG. 35 is a diagram showing Embodiment Mode 3;

FIG. 36 is a diagram showing Embodiment Mode 3;

FIG. 37 is a diagram showing Embodiment Mode 3;

FIG. 38 is a diagram showing Embodiment Mode 3;

FIG. 39 is a diagram showing Embodiment Mode 3;

FIG. 40 is a diagram showing Embodiment Mode 3;

FIG. 41 is a diagram showing Embodiment Mode 3;

FIG. 42 is a diagram showing Embodiment Mode 3;

FIG. 43 is a diagram showing Embodiment Mode 3;

FIG. 44 is a diagram showing Embodiment Mode 3;

FIG. 45 is a diagram showing Embodiment Mode 3;

FIG. 46 is a diagram showing Embodiment Mode 3;

FIG. 47 is a diagram showing Embodiment Mode 3;

FIG. 48 is a diagram showing Embodiment Mode 3;

FIG. 49 is a diagram showing Embodiment Mode 3;

FIG. 50 is a diagram showing Embodiment Mode 3;

FIG. 51 is a diagram showing Embodiment Mode 3;

FIG. 52 is a diagram showing Embodiment Mode 3;

FIG. 53 is a diagram showing Embodiment Mode 3;

FIG. 54 is a diagram showing Embodiment Mode 3;

FIG. 55 is a diagram showing Embodiment Mode 3;

FIG. 56 is a diagram showing Embodiment Mode 3;

FIG. 57 is a diagram showing Embodiment Mode 3;

FIG. 58 is a diagram showing Embodiment Mode 3;

FIG. 59 is a diagram showing Embodiment Mode 3;

FIG. 60 is a diagram showing Embodiment Mode 3;

FIG. 61 is a diagram showing Embodiment Mode 3;

FIG. 62 is a diagram showing Embodiment Mode 3;

FIG. 63 is a diagram showing Embodiment Mode 3;

FIG. 64 is a diagram showing Embodiment Mode 3;

FIG. 65 is a diagram showing Embodiment Mode 3;

FIG. 66 is a diagram showing Embodiment Mode 3;

FIG. 67 is a diagram showing Embodiment Mode 3;

FIG. 68 is a diagram showing Embodiment Mode 3;

FIG. 69 is a diagram showing Embodiment Mode 3;

FIG. 70 is a diagram showing Embodiment Mode 3;

FIG. 71 is a diagram showing Embodiment Mode 3;

FIG. 72 is a diagram showing Embodiment Mode 3;

FIG. 73 is a diagram showing Embodiment Mode 3;

FIG. 74 is a diagram showing Embodiment Mode 3;

FIG. 75 is a diagram showing Embodiment Mode 3;

FIG. 76 is a diagram showing Embodiment Mode 3;

FIG. 77 is a diagram showing Embodiment Mode 3;

FIG. 78 is a diagram showing Embodiment Mode 3;

FIG. 79 is a diagram showing Embodiment Mode 3;

FIG. 80 is a diagram showing Embodiment Mode 3;

FIG. 81 is a diagram showing Embodiment Mode 3;

FIG. 82 is a diagram showing Embodiment Mode 3;

FIG. 83 is a diagram showing Embodiment Mode 3;

FIG. 84 is a diagram showing Embodiment Mode 3;

FIG. 85 is a diagram showing Embodiment Mode 3;

FIG. 86 is a diagram showing Embodiment Mode 3;

FIG. 87 is a diagram showing Embodiment Mode 3;

FIG. 88 is a diagram showing Embodiment Mode 4;

FIG. 89 is a diagram showing Embodiment Mode 4;

FIG. 90 is a diagram showing Embodiment Mode 4;

FIG. 91 is a diagram showing Embodiment Mode 4;

FIG. 92 is a diagram showing Embodiment 1;

FIG. 93 is a diagram showing Embodiment 1;

FIG. 94 is a diagram showing Embodiment 1;

FIG. 95 is a diagram showing Embodiment 2;

FIG. 96 is a diagram showing Embodiment 3;

FIG. 97 is a diagram showing Embodiment 3;

FIG. 98 is a diagram showing Embodiment 3;

FIG. 99 is a diagram showing Embodiment 3;

FIGS. 100A and 100B are diagrams showing Embodiment 4;

FIGS. 101A and 101B are diagrams showing Embodiment 4;

FIGS. 102A and 102B are diagrams showing Embodiment 4;

FIGS. 103A and 103B are diagrams showing Embodiment 4;

FIGS. 104A to 104C are diagrams showing Embodiment 4;

FIG. 105 is a diagram showing Embodiment 4;

FIGS. 106A and 106B are diagrams showing Embodiment 4;

FIGS. 107A and 107B are diagrams showing Embodiment 4;

FIGS. 108A and 108B are diagrams showing Embodiment 4;

FIGS. 109A and 109B are diagrams showing Embodiment 4;

FIGS. 110A and 110B are diagrams showing Embodiment 4;

FIGS. 111A and 111B are diagrams showing Embodiment 4;

FIG. 112 is a diagram showing Embodiment 7;

FIG. 113 is a diagram showing Embodiment 7;

FIGS. 114A and 114B are views showing Embodiment 7;

FIGS. 115A and 115B are diagrams showing Embodiment 7;

FIG. 116 is a view showing Embodiment 6;

FIGS. 117A to 117H are views showing Embodiment 7;

FIG. 118 is a diagram showing Embodiment 3;

FIG. 119 is a diagram showing Embodiment 3;

FIG. 120 is a diagram showing Embodiment 3;

FIG. 121 is a diagram showing Embodiment 3;

FIG. 122 is a diagram showing Embodiment Mode 4;

FIG. 123 is a diagram showing Embodiment Mode 5;

FIG. 124 is a diagram showing Embodiment Mode 3; and

FIG. 125 is a diagram showing Embodiment Mode 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the invention is described below by way of embodiment modesand embodiments with reference to the drawings. However, the inventioncan be implemented by various modes and it is to be understood thatvarious changes and modifications will be apparent to those skilled inthe art. Unless such changes and modifications depart from the spiritand the scope of the invention, they should be construed as beingincluded therein. Therefore, the invention is not limited to thedescription of embodiment modes and embodiments.

Embodiment Mode 1

FIG. 1 shows one mode of a flip-flop circuit 10 of a shift registercircuit of the invention. The shift register circuit of the inventionincludes a plurality stages of the flip-flop circuits 10. The flip-flopcircuit 10 shown in FIG. 1 includes a transistor 11, a transistor 12, atransistor 13, a transistor 14, a transistor 15, a transistor 16, atransistor 17, a transistor 18, and a capacitor 19 having twoelectrodes. However, the capacitor 19 is not necessarily provided in thecase where the gate capacitance of the transistor 12 can be used as thecapacitor 19.

As shown in the flip-flop circuit 10, a gate terminal of the transistor11 is connected to an input terminal IN1. A first terminal of thetransistor 11 is connected to a first power supply. A second terminal ofthe transistor 11 is connected to a gate terminal of the transistor 12,a second terminal of the transistor 14, a gate terminal of thetransistor 15, a second terminal of the transistor 17, and a secondelectrode of the capacitor 19. A first terminal of the transistor 15 isconnected to a second power supply, and a second terminal of thetransistor 15 is connected to a second terminal of the transistor 16 anda gate terminal of the transistor 18. A gate terminal and a firstterminal of the transistor 16 are connected to the first power supply. Afirst terminal of the transistor 18 is connected to an input terminalIN3, and a second terminal of the transistor 18 is connected to a gateterminal of the transistor 13 and a gate terminal of the transistor 14.A first terminal of the transistor 13 is connected to the second powersupply. A second terminal of the transistor 13 is connected to a firstelectrode of the capacitor 19, a second terminal of the transistor 12,and an output terminal OUT. A first terminal of the transistor 12 isconnected to an input terminal IN2. A first terminal of the transistor14 is connected to the second power supply. A gate terminal of thetransistor 17 is connected to an input terminal IN4, and a firstterminal of the transistor 17 is connected to the second power supply.

It is to be noted that in the flip-flop circuit 10, a node of the secondterminal of the transistor 11, the gate terminal of the transistor 12,the second terminal of the transistor 14, the gate terminal of thetransistor 15, the second terminal of the transistor 17, and the secondelectrode of the capacitor 19 is denoted by N1. A node of the secondterminal of the transistor 15, the second terminal of the transistor 16and the gate terminal of the transistor 18 is denoted by N2. A node ofthe gate terminal of the transistor 13, the gate terminal of thetransistor 14, and the second terminal of the transistor 18 is denotedby N3.

In addition, a power supply potential VDD is supplied to the first powersupply, and a power supply potential VSS is supplied to the second powersupply. A potential difference (VDD−VSS) between the power supplypotential VDD of the first power supply and the power supply potentialVSS of the second power supply corresponds to a power supply voltage ofthe flip-flop circuit 10. Further, the power supply potential VDD ishigher than the power supply potential VSS.

Further, a control signal is supplied to each of the input terminals IN1to IN4. In addition, the output terminal OUT outputs an output signal.An output signal of a flip-flop circuit 10 in the previous stage issupplied to the input terminal IN1 as the control signal. An outputsignal of a flip-flop circuit 10 in the next stage is supplied to theinput terminal IN4 as the control signal.

Moreover, each of the transistors 11 to 18 is an n-channel transistor.However, each of the transistors 11 to 18 may be a p-channel transistor.

Next, an operation of the flip-flop circuit 10 shown in FIG. 1 isdescribed with reference to a timing chart shown in FIG. 2. FIG. 2 is atiming chart of the control signal which is supplied to each of theinput terminals N1 to 1N4, the output signal which is output from theoutput terminal OUT, and potentials of the nodes N1 to N3 shown inFIG. 1. The timing chart shown in FIG. 2 is divided into a period T1 toa period T4 for convenience.

It is to be noted that in periods after the period T4, the period T3 andthe period T4 are sequentially repeated. In addition, in FIG. 2, theperiod T1 is defined as a selection preparation period; the period T2 isdefined as a selection period; and the period T3 and the period T4 aredefined as non-selection periods. That is, one selection preparationperiod, one selection period, and a plurality of non-selection periodsare sequentially repeated.

In addition, in the timing chart shown in FIG. 2, each of the controlsignal and the output signal has two values. That is, each of thesesignals is a digital signal. One of the potentials of the digital signalis VDD which is the same potential as the first power supply potential(hereinafter also described as a potential VDD or an H level) when thedigital signal is an H signal, and the other of the potentials of thedigital signal is VSS which is the same potential as the second powersupply potential (hereinafter also described as a potential VSS or an Llevel) when the digital signal is an L signal.

Further, FIGS. 3 to 6 show connection states of the flip-flop circuits10 corresponding to operations in the period T1 to the period T4,respectively.

Moreover, in FIGS. 3 to 6, transistors shown in solid lines are on andtransistors shown in broken lines are off. Wirings shown in solid linesare connected to power supplies or input terminals, and wirings shown inbroken line are not connected to the power supplies or the inputterminals.

Next, the operation in each period is described with reference to FIGS.3 to 6.

First, an operation of the flip-flop circuit 10 in the period T1 isdescribed with reference to FIG. 3. FIG. 3 is a diagram showing aconnection state of the flip-flop circuit 10 in the period T1.

In the period T1, the input terminal IN1 becomes an H level to turn onthe transistor 11, and the input terminal IN4 becomes an L level to turnoff the transistor 17. Since the node N3 is held at VSS obtained in theperiod T3 which is described later, the transistor 14 is turned off. Thenode N1 is electrically connected to the first power supply through thetransistor 11, and a potential of the node N1 rises to be Vn11. When thenode N1 becomes Vn11, the transistor 11 is turned off. Here, Vn11 is avalue obtained by subtracting the threshold voltage Vth11 of thetransistor 11 from the power supply potential VDD (VDD−Vth11). Note thatVn11 is a potential which can turn on the transistor 12 and thetransistor 15.

When the potential of the node N1 becomes Vn11, the transistor 11 isturned off and the transistor 12 and the transistor 15 are turned on.The node N2 is electrically connected to the second power supply throughthe transistor 15 and is electrically connected to the first powersupply through the transistor 16, and a potential of the node N2 risesto be Vn21. Here, Vn21 is determined by an operating point of thetransistor 16 and the transistor 15. Note that the transistor 15 and thetransistor 16 form an inverter using the two transistors. Accordingly,when an H-level signal is input into the gate terminal of the transistor15 (the node N1), an L-level signal is input into the node N2. Here,Vn21 is a potential which can turn off the transistor 18. Accordingly,since the transistor 18 is off even when the input terminal IN3 is at anH level, the node N3 can be held at VSS. Since the input terminal IN2becomes an L level, and the output terminal OUT is electricallyconnected to the input terminal IN2 through the transistor 12, apotential of the output terminal OUT becomes VSS.

Since the potential of the node N2 becomes Vn21 and the transistor 18 isoff, the node N3 is held at VSS and the transistor 13 and the transistor14 are turned off.

By the above-described operations, the transistor 12 is on and theoutput terminal OUT is set at an L level in the period T1. In addition,since the transistor 11 is off, the node N1 is set in a floating sate.

Next, an operation of the flip-flop circuit 10 in the period T2 isdescribed with reference to FIG. 4. FIG. 4 is a diagram showing aconnection state of the flip-flop circuit 10 in the period T2.

In the period T2, the input terminal IN1 becomes an L level and thetransistor 11 is off. The input terminal IN4 is unchanged at an L leveland the transistor 17 is off. Therefore, the node N1 is kept in afloating state from the period T1 to hold the potential Vn11 in theperiod T1.

Since the potential of the node N1 is held at Vn11, the transistor 12 ison. The input terminal IN2 becomes an H level. Then, since the outputterminal OUT is electrically connected to the input terminal IN2 throughthe transistor 12, the potential of the output terminal OUT rises fromVSS. The potential of the node N1 is changed into Vn12 by the capacitivecoupling of the capacitor 19 to keep the on state of the transistor 12.A so-called bootstrap operation is performed. Accordingly, the potentialof the output terminal OUT rises to a potential equal to VDD which is apotential of the input terminal IN2. Note that Vn12 is a value which isgreater than or equal to the sum of the potential VDD and the thresholdvoltage Vth12 of the transistor 12.

The transistor 15 is continuously kept on even when the potential of thenode N1 becomes Vn12. Therefore, the potential of the node N2 and apotential of the node N3 have the same potentials as those in the periodT1.

By the above-described operations, the potential of the node N1 which isin a floating state is raised by the bootstrap operation, so that thetransistor 12 is continuously kept on in the period T2. Thus, thepotential of the output terminal OUT is set at VDD so that the outputterminal OUT has an H level.

Next, an operation of the flip-flop circuit 10 in the period T3 isdescribed with reference to FIG. 5. FIG. 5 is a diagram showing aconnection state of the flip-flop circuit 10 in the period T3.

In the period T3, the input terminal IN1 is unchanged at an L level andthe transistor 11 is off. The input terminal IN4 becomes an H level toturn on the transistor 17. Then, the node N1 is electrically connectedto the second power supply through the transistor 17 so that thepotential of the node N1 becomes VSS.

The potential of the node N1 becomes VSS to turn off the transistor 12and the transistor 15. Since the node N2 is electrically connected tothe first power supply through the transistor 16, the potential of thenode N2 rises to be Vn22. Here, Vn22 is a value obtained by subtractingthe threshold voltage Vth16 of the transistor 16 from the power supplypotential VDD (VDD−Vth16). Note that Vn22 is a potential which can turnon the transistor 18.

When the potential of the node N2 becomes Vn22, the transistor 18 isturned on. Then, since the input terminal IN3 becomes an H level, thenode N3 is electrically connected to the input terminal IN3 through thetransistor 18 and a potential of the node N3 becomes Vn31. Here, Vn31 isa value obtained by subtracting the threshold voltage Vth18 of thetransistor 18 from Vn22 which is the potential of the node N2(Vn22−Vth18). Note that Vn31 corresponds to a value obtained bysubtracting the threshold voltage Vth16 of the transistor 16 and thethreshold voltage Vth18 of the transistor 18 from the power supplypotential VDD (VDD−Vth16−Vth18). Note that Vn31 is a potential which canturn on the transistor 13 and the transistor 14.

When the potential of the node N3 becomes Vn31, the transistor 13 isturned on. Then, since the output terminal OUT is electrically connectedto the second power supply through the transistor 13, the potential ofthe output terminal OUT becomes VSS.

By the above-described operations, VSS is supplied to the node N1 toturn off the transistor 12 and the transistor 15 in the period T3. Inaddition, the node N3 is set at an H level to turn on the transistor 13and the transistor 14. Accordingly, the potential of the output terminalOUT is set at VSS so that the output terminal OUT has an L level.

Next, an operation of the flip-flop circuit 10 in the period T4 isdescribed with reference to FIG. 6. FIG. 6 is a diagram showing aconnection state of the flip-flop circuit 10 in the period T4.

In the period T4, the input terminal IN3 becomes an L level and thepotential of the node N3 becomes VSS. Thus, the transistor 13 and thetransistor 14 are turned off. The input terminal IN4 becomes an L levelto turn off the transistor 17. Therefore, the node N1 becomes a floatingstate and the potential of the node N1 is held at VSS.

Since the potential of the node N1 is unchanged at VSS, the transistor12 and the transistor 15 are continuously kept off. Accordingly, thenode N2 is continuously kept at Vn22 and the transistor 18 iscontinuously kept on.

Since the transistor 12 and the transistor 13 are turned off, the outputterminal OUT becomes a floating state. Thus, the potential of the outputterminal OUT is held at VSS.

By the above-described operations, the potential of the output terminalOUT is held at VSS so that the transistor 13 and the transistor 14 canbe turned off in the period T4. Since the transistor 13 and thetransistor 14 are not always on, characteristic deterioration of thetransistor 13 and the transistor 14 can be suppressed.

Relations among the period T1 to the period T4 are described. The nextperiod of the period T1 is the period T2; the next period of the periodT2 is the period T3; and next period of the period T3 is the period T4.Here, the next period of the period T4 is the period T1 or the periodT3. That is, the next period of the period T4 is the period T1 when theinput terminal IN1 becomes an H level, or the next period of the periodT4 is the period T3 when the input terminal IN1 is unchanged at an Llevel. In addition, when the period T3 is the next period of the periodT4, the input terminal IN4 is unchanged at an L level and the transistor17 is continuously kept off.

Here, functions of the transistors 11 to 18 and the capacitor 19 aredescribed below.

The transistor 11 has a function as a switch which selects whether toconnect the first power supply and the node N1 or not in accordance withthe control signal which is supplied to the input terminal IN1. In theperiod T1, the transistor 11 has functions of supplying the power supplypotential VDD to the node N1 and being turned off when the potential ofthe node N1 becomes Vn11.

In addition, the transistor 11 has a function of making the node N1 intoa floating state in accordance with the control signal which is suppliedto the input terminal IN1. In the period T1 and the period T2, thetransistor 11 has a function of being turned off when the potential ofthe node N1 becomes greater than or equal to Vn11.

The transistor 12 has a function as a switch which selects whether toconnect the input terminal IN2 and the output terminal OUT or not inaccordance with the potential of the node N1. In the period T1, thetransistor 12 has a function of supplying VSS to the output terminalOUT. In the period T2, the transistor 12 has a function of supplying VDDto the output terminal OUT.

The transistor 13 has a function as a switch which selects whether toconnect the second power supply and the output terminal OUT or not inaccordance with the potential of the node N3. In the period T3, thetransistor 13 has a function of supplying the power supply potential VSSto the output terminal OUT.

The transistor 14 has a function as a switch which selects whether toconnect the second power supply and the node N1 or not in accordancewith the potential of the node N3. In the period T3, the transistor 14has a function of supplying the power supply potential VSS to the nodeN1.

The transistor 15 has a function as a switch which selects whether toconnect the second power supply and the node N2 or not in accordancewith the potential of the node N1. In the period T1 and the period T2,the transistor 15 has a function of supplying the power supply potentialVSS to the node N2.

The transistor 16 has a function as a diode having an input terminalconnected to the first power supply and an output terminal connected tothe node N2.

The transistor 17 has a function as a switch which selects whether toconnect the second power supply and the node N1 or not in accordancewith the control signal which is supplied to the input terminal IN4. Inthe period T3 which is after the period T2, the transistor 17 has afunction of supplying the power supply voltage VSS to the node N1.

The transistor 18 has a function as a switch which selects whether toconnect the input terminal IN3 and the node N3 or not in accordance withthe potential of the node N2. In the period T3, the transistor 18 has afunction of supplying VDD to the node N3. In the period T4, thetransistor 18 has a function of supplying VSS to the node N3.

The capacitor 19 has a function for changing the potential of the nodeN1 in accordance with the potential of the output terminal OUT. In theperiod T2, the capacitor 19 has a function of raising the potential ofthe node N1 by the rise of the potential of the output terminal OUT.

In this manner, in the flip-flop circuit 10 shown in FIG. 1, thetransistor 13 and the transistor 14 are turned on in the period T3 andturned off in the period T4, so that the transistor 13 and thetransistor 14 can be prevented from always being on. Accordingly,characteristic deterioration of the transistor 13 and the transistor 14can be suppressed. Therefore, in the flip-flop circuit 10 shown in FIG.1, a malfunction due to the characteristic deterioration of thetransistor 13 and the transistor 14 can also be suppressed.

In addition, when the transistor 13 and the transistor 14 are turned on,the power supply potential VSS is supplied to the output terminal OUTand the node N1. Therefore, in the flip-flop circuit 10 shown in FIG. 1,the power supply potential VSS can be supplied to the output terminalOUT and the node N1 at regular intervals, so that fluctuation in thepotentials of the output terminal OUT and the node N1 can be suppressed.

Further, the flip-flop circuit 10 shown in FIG. 1 is formed by using alln-channel transistors so that amorphous silicon can be used as asemiconductor layer. Thus, a manufacturing process can be simplified, sothat a manufacturing cost can be reduced and yield can be improved. Inaddition, a large display panel can be made. Furthermore, by using theflip-flop circuit of the invention, the life of the semiconductor devicecan be extended even in the case of using a transistor made fromamorphous silicon, characteristics of which easily deteriorate.

It is to be noted that in the period T1 to the period T4, elements suchas transistors or switches may be provided in the flip-flop circuits 10so as to satisfy the states in FIGS. 3 to 6, respectively.

It is to be noted that the capacitor 19 is preferably formed by using agate wiring layer and a semiconductor layer. The gate wiring layer andthe semiconductor layer are stacked with a gate insulating filminterposed therebetween. Since the film thickness of the gate insulatingfilm is much thinner than other insulating layers such as an interlayerfilm, the capacitor can have a small area and high capacity when thegate insulating film is used as an insulator.

In addition, the size (W/L) of the transistor 15 is preferably largerthan that of the transistor 16. Here, W means the channel width of atransistor and L means the channel length of the transistor. When thetransistor 15 is turned on, the potential of the node N2 is determinedby the operating point of the transistor 15 and the transistor 16. Thatis, if the size of the transistor 15 is not sufficiently larger thanthat of the transistor 16, the potential of the node N2 becomes higher,so that the transistor 18 cannot be turned off. Accordingly, in order toturn off the transistor 18, the size of the transistor 15 should besufficiently larger than that of the transistor 16.

In addition, the size of the transistor 15 is preferably four times aslarge as that of the transistor 16 or more. More preferably, the size ofthe transistor 15 is ten times as large as that of the transistor 16 ormore. When the power supply voltage is low, the ratio of the sizes ofthe transistor 15 to the transistor 16 may be approximately 4:1.However, when the power supply voltage becomes higher, the ratio of thesizes of the transistor 15 to the transistor 16 should be approximately10:1.

Here, when a level-shift circuit or the like is connected to the outputterminal OUT of the flip-flop circuit 10, the ratio of the sizes of thetransistor 15 to the transistor 16 is preferably 4:1 or more. This isbecause the amplitude voltage of an output signal of the flip-flopcircuit 10 is increased by the level-shift circuit or the like, so thatthe flip-flop circuit 10 often operates with a low power supply voltage.

Alternatively, when the level-shift circuit or the like is not connectedto the output terminal OUT of the flip-flop circuit 10, the ratio of thesizes of the transistor 15 to the transistor 16 is preferably 10:1 ormore. This is because the output signal of the flip-flop circuit 10 isapplied to some kind of operation without being level shifted, so thatthe flip-flop circuit 10 often operates with a high power supplyvoltage.

Note that each of the power supply potentials and potentials of thecontrol signals may be any potential as long as it can control on/off ofa target transistor.

For example, the power supply potential VDD may be higher than anH-level potential of a control signal. This is because the potential ofthe node N3 is Vn31 (VDD−Vth16−Vth18), so that Vn31 which is thepotential of the node N3 becomes higher when the power supply potentialVDD becomes higher. Accordingly, the transistor 13 and the transistor 14can be surely turned on even when the threshold voltages of thetransistor 13 and the transistor 14 become higher due to thecharacteristic deterioration of the transistor 13 and the transistor 14.

In addition, the power supply potential VDD may be a potential lowerthan the H-level potential of the control signal as long as it cancontrol on/off of each transistor.

Note that the capacitor 19 is not necessarily provided when gatecapacitance (parasitic capacitance) between the gate terminal and thesecond terminal of the transistor 12 is sufficiently large.

For example, the capacitor 19 is not necessary connected as in aflip-flop circuit 70 in FIG. 7. Accordingly, since the number ofelements in the flip-flop circuit 70 is one less than the number ofelements in the flip-flop circuit 10, each element can be arranged inhigh density in the flip-flop circuit 70.

In addition, as another example, a capacitor may be formed by using atransistor 101 as in a flip-flop circuit 100 in FIG. 10. This is becausethe gate capacitance of the transistor 101 sufficiently functions as acapacitor when the transistor 101 is on.

It is to be noted that since the transistor 101 is on in the period T1and the period T2 (at the time of performing the bootstrap operation), achannel region is formed in the transistor 101 so that the transistor101 functions as the capacitor. On the other hand, since the transistor101 is off in the period T3 and the period T4 (at the time of notperforming the bootstrap operation), a channel region is not formed inthe transistor 101, so that the transistor 101 does not function as thecapacitor or functions as a small capacitor.

Here, by forming the capacitor by using the transistor 101 as in theflip-flop circuit 100 in FIG. 10 which is described above, thetransistor 101 functions as the capacitor only when needed (in theperiod T1 and the period T2), and the transistor 101 does not functionas the capacitor when not needed (in the period T3 and the period T4).Therefore, the flip-flop circuit 100 hardly malfunctions due to changesin the potentials of the node N1 and the output terminal OUT.

Note that the transistor 101 has the same polarity as that of thetransistor 12.

It is also to be noted that the first terminal of the transistor 11 maybe connected anywhere in the period T1 and the period T2 as long as itcan make the node N1 into a floating state.

For example, the first terminal of the transistor 11 may be connected tothe input terminal IN1 as in a flip-flop circuit 80 in FIG. 8. This isbecause the node N1 can be made into a floating state in the period T1and the period T2 even when the first terminal of the transistor 11 isconnected to the input terminal IN1.

Note that in the flip-flop circuit 10 in FIG. 1, noise is generated inthe first power supply by parasitic capacitance between the firstterminal and the gate terminal of the transistor 11 when the potentialof the input terminal IN1 is changed. In addition, when a current issupplied from the first power supply to the node N1 by on/off of thetransistor 11, noise is generated in the first power supply by a voltagedrop due to the current. Such noise is generated by changes in thepotential of the input terminal IN1.

Here, by connecting as in the flip-flop circuit 80 in FIG. 8 which isdescribed above, the above-described noise can be suppressed. Inaddition, by suppressing the noise in the first power supply, anothercircuit using the first power supply can operate stably.

It is to be noted that another circuit using the first power supplycorresponds to an inverter circuit, a level-shift circuit, a latchcircuit, a PWC circuit, or the like which is connected to the outputterminal OUT of the flip-flop circuit 80.

Note also that any element can be used as the transistor 16 as long asit can form an inverter circuit with the transistor 15. The transistor16 does not necessarily have rectifying properties; any element can beused as long as a voltage is generated in the element when a current issupplied thereto.

For example, a resistor 91 may be connected as a substitute for thetransistor 16 as in a flip-flop circuit 90 in FIG. 9. This is because aninverter circuit can be formed by using the resistor 91 and thetransistor 15 even when the resistor 91 is connected as a substitute forthe transistor 16.

Note that when the transistor 15 is off, the potential of the node N2becomes VDD which is the same potential as that of the first powersupply. In addition, the potential of the node N3 at this time becomes avalue obtained by subtracting the threshold voltage Vth18 of thetransistor 18 from the power supply potential VDD (VDD−Vth18).

Here, by using the resistor 91 as a substitute for the transistor 16 asin the flip-flop circuit 90 in FIG. 9 which is described above, thepotential of the node N2 becomes VDD and the potential of the node N3only becomes lower than VDD by the threshold voltage Vth18 of thetransistor 18 even when the threshold voltage of each transistor becomeshigher due to characteristic deterioration, and thus, the transistor 13and the transistor 14 can be easily turned on.

It is to be noted that although a control signal is supplied to each ofthe input terminal IN1, the input terminal IN2, the input terminal IN3,and the input terminal IN4, the invention is not limited to this.

For example, each of the input terminal IN1, the input terminal IN2, theinput terminal IN3, and the input terminal IN4 may be supplied with thepower supply potential VDD, the power supply potential VSS, or anotherpotential.

It is to be noted that although the first terminal of the transistor 11and the first terminal of the transistor 16 are connected to the firstpower supply, the invention is not limited to this.

For example, the first terminal of the transistor 11 and the firstterminal of the transistor 16 may be connected to different powersupplies, respectively. In that case, a potential of a power supplyconnected to the first terminal of the transistor 16 is preferablyhigher than a potential of a power supply connected to the firstterminal of the transistor 11.

As another example, a control signal may be supplied to each of thefirst terminal of the transistor 11 and the first terminal of thetransistor 16.

It is to be noted that although the first terminal of the transistor 13,the first terminal of the transistor 14, and the first terminal of thetransistor 17 are connected to the second power supply, the invention isnot limited to this.

For example, the first terminal of the transistor 13, the first terminalof the transistor 14, and the first terminal of the transistor 17 may beconnected to different power supplies, respectively.

As another example, a control signal may be supplied to each of thefirst terminal of the transistor 13, the first terminal of thetransistor 14, and the first terminal of the transistor 17.

Although the flip-flop circuit 10 shown in FIG. 1 is formed by using alln-channel transistors, the flip-flop circuit 10 shown in FIG. 1 may beformed by using all p-channel transistors as well. Here, a flip-flopcircuit which is formed by using transistors which are all p-channeltransistors is shown in FIG. 11.

FIG. 11 shows one mode of a flip-flop circuit 110 of the shift registercircuit of the invention. The shift register circuit of the inventionincludes a plurality of the flip-flop circuits 110. The flip-flopcircuit 110 shown in FIG. 11 includes a transistor 111, a transistor112, a transistor 113, a transistor 114, a transistor 115, a transistor116, a transistor 117, a transistor 118, and a capacitor 119 having twoelectrodes. However, the capacitor 119 is not necessarily provided inthe case where the gate capacitance of the transistor 112 can be used asa substitute for the capacitor 119.

As shown in the flip-flop circuit 110, a gate terminal of the transistor111 is connected to the input terminal IN1. A first terminal of thetransistor 111 is connected to the first power supply. A second terminalof the transistor 111 is connected to a gate terminal of the transistor112, a second terminal of the transistor 114, a gate terminal of thetransistor 115, a second terminal of the transistor 117, and a secondelectrode of the capacitor 119. A first terminal of the transistor 115is connected to the second power supply, and a second terminal of thetransistor 115 is connected to a second terminal of the transistor 116and a gate terminal of the transistor 118. A gate terminal and a firstterminal of the transistor 116 are connected to the first power supply.A first terminal of the transistor 118 is connected to the inputterminal IN3, and a second terminal of the transistor 118 is connectedto a gate terminal of the transistor 113 and a gate terminal of thetransistor 114. A first terminal of the transistor 113 is connected tothe second power supply. A second terminal of the transistor 113 isconnected to a first electrode of the capacitor 119, a second terminalof the transistor 112, and the output terminal OUT. A first terminal ofthe transistor 112 is connected to the input terminal IN2. A firstterminal of the transistor 114 is connected to the second power supply.A gate terminal of the transistor 117 is connected to the input terminalIN4, and a first terminal of the transistor 117 is connected to thesecond power supply.

It is to be noted that in the flip-flop circuit 110, a node of thesecond terminal of the transistor 111, the gate terminal of thetransistor 112, the second terminal of the transistor 114, the gateterminal of the transistor 115, the second terminal of the transistor117, and the second electrode of the capacitor 119 is denoted by N1. Anode of the second terminal of the transistor 115, the second terminalof the transistor 116 and the gate terminal of the transistor 118 isdenoted by N2. A node of the gate terminal of the transistor 113, thegate terminal of the transistor 114, and the second terminal of thetransistor 118 is denoted by N3.

In addition, the power supply potential VSS is supplied to the firstpower supply, and the power supply potential VDD is supplied to thesecond power supply. A potential difference (VDD−VSS) between the powersupply potential VSS of the first power supply and the power supplypotential VDD of the second power supply corresponds to a power supplyvoltage of the flip-flop circuit 110. The power supply potential VDD ishigher than the power supply potential VSS.

Further, a control signal is supplied to each of the input terminals IN1to IN4. In addition, the output terminal OUT outputs an output signal.An output signal of a flip-flop circuit 110 in the previous stage issupplied to the input terminal IN1 as the control signal. An outputsignal of a flip-flop circuit 110 in the next stage is supplied to theinput terminal IN4 as the control signal.

Moreover, each of the transistors 111 to 118 is a p-channel transistor.However, each of the transistors 111 to 118 may be an n-channeltransistor.

Next, an operation of the flip-flop circuit 110 shown in FIG. 11 isdescribed with reference to a timing chart shown in FIG. 12. FIG. 12 isa timing chart of the control signal which is supplied to each of theinput terminals IN1 to IN4, the output signal which is output from theoutput terminal OUT, and potentials of the nodes N1 to N3 shown in FIG.11. Note that with respect to the timing of the control signal and theoutput signal, an H level and an L level are inverted from those in thecase where the flip-flop circuit is formed by using all n-channeltransistors (FIG. 1). The timing chart shown in FIG. 12 is divided intoa period T1 to a period T4 for convenience.

It is to be noted that in periods after the period T4, the period T3 andthe period T4 are sequentially repeated. In addition, in FIG. 12, theperiod T1 is defined as a selection preparation period; the period T2 isdefined as a selection period; and the period T3 and the period T4 aredefined as non-selection periods. That is, one selection preparationperiod, one selection period, and a plurality of non-selection periodsare sequentially repeated.

In addition, in the timing chart shown in FIG. 12, each of the controlsignal and the output signal is a digital signal having two values. Oneof the two values of the digital signal is VDD which is the samepotential as the second power supply potential (hereinafter alsodescribed as a potential VDD or an H level) when the digital signal isan H signal, and the other of the two values of the digital signal isVSS which is the same potential as the first power supply potential(hereinafter also described as a potential VSS or an L level) when thedigital signal is an L signal.

Next, operations of the flip-flop circuit 110 in each period aredescribed.

First, an operation of the flip-flop circuit 110 in the period T1 isdescribed.

In the period T1, the input terminal IN1 becomes an L level to turn onthe transistor 111, and the input terminal IN4 becomes an H level toturn off the transistor 117. Since the node N3 is held at VDD obtainedin the period T3 which is described later, the transistor 114 is turnedoff. The node N1 is electrically connected to the first power supplythrough the transistor 111, and a potential of the node N1 lowers to beVn11. When the node N1 becomes Vn11, the transistor 111 is turned off.Here, Vn11 is a value which is the sum of the power supply potential VSSand the absolute value of the threshold voltage Vth111 of the transistor111 (VSS+|Vth111|). Note that Vn111 is a potential which can turn on thetransistor 112 and the transistor 115.

When the potential of the node N1 becomes Vn111, the transistor 111 isturned off and the transistor 112 and the transistor 115 are turned on.The node N2 is electrically connected to the second power supply throughthe transistor 115 and is electrically connected to the first powersupply through the transistor 116, and a potential of the node N2becomes Vn21. Here, Vn21 is determined by an operating point of thetransistor 116 and the transistor 115. Note that the transistor 115 andthe transistor 116 form an inverter using the two transistors.Accordingly, when an L-level signal is input into the gate terminal ofthe transistor 115 (the node N1), an H-level signal is input into thenode N2. Here, Vn21 is a potential which can turn off the transistor118. Accordingly, since the transistor 118 is off even when the inputterminal IN3 is at an L level, the node N3 can be held at VDD. Since theinput terminal IN2 becomes an H level, and the output terminal OUT iselectrically connected to the input terminal IN2 through the transistor112, the potential of the output terminal OUT becomes VDD.

Since the potential of the node N2 becomes Vn21 and the transistor 118is off, the node N3 is held at VDD and the transistor 113 and thetransistor 114 are turned off.

By the above-described operations, the transistor 112 is on and theoutput terminal OUT is set at an H level in the period T1. In addition,since the transistor 111 is off, the node N1 is set in a floating sate.

Next, an operation of the flip-flop circuit 110 in the period T2 isdescribed.

In the period T2, the input terminal IN1 becomes an H level and thetransistor 111 is off. The input terminal IN4 is unchanged at an H leveland the transistor 117 is off. Therefore, the node N1 is kept in afloating state from the period T1 to hold the potential Vn11 in theperiod T1.

Since the potential of the node N1 is held at Vn11, the transistor 112is on. The input terminal IN2 becomes an L level. Then, since the outputterminal OUT is electrically connected to the input terminal IN2 throughthe transistor 112, the potential of the output terminal OUT lowers fromVDD. The potential of the node N1 is changed into Vn12 by the capacitivecoupling of the capacitor 119 to keep the on state of the transistor112. A so-called bootstrap operation is performed. Accordingly, thepotential of the output terminal OUT lowers to a potential equal to VSSwhich is a potential of the input terminal IN2. Note that Vn12 is avalue which is less than or equal to a value obtained by subtracting theabsolute value of the threshold voltage Vth112 of the transistor 112from the potential VSS (VSS−|Vth112|). Since the input terminal IN2becomes an L level, and the output terminal OUT is electricallyconnected to the input terminal IN2 through the transistor 112, thepotential of the output terminal OUT becomes VSS.

The transistor 115 is continuously kept on even when the potential ofthe node N1 becomes Vn12. Therefore, the potential of the node N2 andthe potential of the node N3 have the same potentials as those in theperiod T1.

By the above-described operations, the potential of the node N1 which isin a floating state is lowered by the bootstrap operation, so that theoutput terminal OUT has VSS.

Next, an operation of the flip-flop circuit 110 in the period T3 isdescribed.

In the period T3, the input terminal IN1 is unchanged at an H level andthe transistor 111 is off. The input terminal IN4 becomes an L level toturn on the transistor 117. Then, the node N1 is electrically connectedto the second power supply through the transistor 117 so that thepotential of the node N1 becomes VDD.

The potential of the node N1 becomes VDD to turn off the transistor 112and the transistor 115. Since the node N2 is electrically connected tothe first power supply through the transistor 116, the potential of thenode N2 lowers to be Vn22. Here, Vn22 is a value which is the sum of thepower supply potential VSS and the absolute value of the thresholdvoltage Vth116 of the transistor 116 (VSS+|Vth116|). Note that Vn22 is apotential which can turn on the transistor 118.

When the potential of the node N2 becomes Vn22, the transistor 118 isturned on. Then, since the input terminal IN3 becomes an L level, thenode N3 is electrically connected to the input terminal IN3 through thetransistor 118 and the potential of the node N3 becomes Vn31. Here, Vn31is a value which is the sum of Vn22 which is the potential of the nodeN2 and the absolute value of the threshold voltage Vth118 of thetransistor 118 (Vn22+|Vth118|). Note that Vn31 corresponds to a valuewhich is the sum of the power supply potential VSS, the absolute valueof the threshold voltage Vth116 of the transistor 116, and the absolutevalue of the threshold voltage Vth118 of the transistor 118(VSS+|Vth116|+|Vth118|). In addition, Vn31 is a potential which can turnon the transistor 113 and the transistor 114.

When the potential of the node N3 becomes Vn31, the transistor 113 isturned on. Then, since the output terminal OUT is electrically connectedto the second power supply through the transistor 113, the potential ofthe output terminal OUT becomes VDD.

By the above-described operations, VDD is supplied to the node N1 toturn off the transistor 112 and the transistor 115 in the period T3. Inaddition, the node N3 is set at an L level to turn on the transistor 113and the transistor 114. Accordingly, the potential of the outputterminal OUT is set at VDD so that the output terminal OUT has an Hlevel.

Next, an operation of the flip-flop circuit 110 in the period T4 isdescribed.

In the period T4, the input terminal IN3 becomes an H level and thepotential of the node N3 becomes VDD. Thus, the transistor 113 and thetransistor 114 are turned off. The input terminal IN4 becomes an H levelto turn off the transistor 117. Therefore, the node N1 becomes afloating state and the potential of the node N1 is held at VDD.

Since the potential of the node N1 is unchanged at VDD, the transistor112 and the transistor 115 are continuously kept off. Accordingly, thenode N2 is unchanged at Vn22 and the transistor 118 is continuously kepton.

Since the transistor 112 and the transistor 113 are turned off, theoutput terminal OUT becomes a floating state. Thus, the potential of theoutput terminal OUT is held at VDD.

By the above-described operations, the potential of the output terminalOUT is held at VDD so that the transistor 113 and the transistor 114 canbe turned off in the period T4. Since the transistor 113 and thetransistor 114 are not always on, characteristic deterioration of thetransistor 113 and the transistor 114 can be suppressed.

Relations among the period T1 to the period T4 are described. The nextperiod of the period T1 is the period T2; the next period of the periodT2 is the period T3; and next period of the period T3 is the period T4.Here, the next period of the period T4 is the period T1 or the periodT3. That is, the next period of the period T4 is the period T1 when theinput terminal IN1 becomes an L level, or the next period of the periodT4 is the period T3 when the input terminal IN1 is unchanged at an Hlevel. In addition, when the period T3 is the next period of the periodT4, the input terminal IN4 is unchanged at an H level and the transistor117 is continuously kept off.

Here, the transistor 111 to the transistor 118, and the capacitor 119have the same functions as those of the transistor 11 to the transistor18, and the capacitor 19 shown in FIG. 1, respectively.

In this manner, in the flip-flop circuit 110 shown in FIG. 11, thetransistor 113 and the transistor 114 are turned on in the period T3 andturned off in the period T4, so that the transistor 113 and thetransistor 114 can be prevented from always being on. Accordingly, thecharacteristic deterioration of the transistor 113 and the transistor114 can be suppressed. Therefore, in the flip-flop circuit 110 shown inFIG. 11, a malfunction due to the characteristic deterioration of thetransistor 113 and the transistor 114 can also be suppressed.

In addition, when the transistor 113 and the transistor 114 are turnedon, the power supply potential VDD is supplied to the output terminalOUT and the node N1. Therefore, in the flip-flop circuit 110 shown inFIG. 11, the power supply potential VDD can be supplied to the outputterminal OUT and the node N1 at regular intervals, so that fluctuationin the potentials of the output terminal OUT and the node N1 can besuppressed.

Further, in the flip-flop circuit 110 shown in FIG. 11, polysilicon canbe used as a semiconductor layer, so that a manufacturing process can besimplified. Thus, a manufacturing cost can be reduced and yield can beimproved. Furthermore, since characteristics in polysilicon hardlydeteriorate, the life of the semiconductor device can be more extendedthan the case of using amorphous silicon as the semiconductor layer. Byusing the flip-flop circuit of the invention, the life of thesemiconductor device can be more extended. Moreover, since the mobilityof a transistor using polysilicon is high, the flip-flop circuit 110 canoperate at high speed.

It is to be noted that the capacitor 119 is preferably formed by using agate wiring layer and a semiconductor layer. The gate wiring layer andthe semiconductor layer are stacked with a gate insulating filminterposed therebetween. Since the film thickness of the gate insulatingfilm is much thinner than other insulating layers such as an interlayerfilm, the capacitor can have a small area and high capacity when thegate insulating film is used as an insulator.

In addition, the size (W/L) of the transistor 115 is preferably largerthan that of the transistor 116. Here, W means the channel width of atransistor and L means the channel length of the transistor. When thetransistor 115 is turned on, the potential of the node N2 is determinedby the operating point of the transistor 115 and the transistor 116.That is, if the size of the transistor 115 is not sufficiently largerthan that of the transistor 116, the potential of the node N2 becomeshigher, so that the transistor 118 cannot be turned off. Accordingly, inorder to turn off the transistor 118, the size of the transistor 115should be sufficiently larger than that of the transistor 116.

In addition, the size of the transistor 115 is preferably four times aslarge as that of the transistor 116 or more. More preferably, the sizeof the transistor 115 is ten times as large as that of the transistor116 or more. When the power supply voltage is low, the ratio of thesizes of the transistor 115 to the transistor 116 may be approximately4:1. However, when the power supply voltage becomes higher, the ratio ofthe sizes of the transistor 115 to the transistor 116 should beapproximately 10:1.

Here, when a level-shift circuit or the like is connected to the outputterminal OUT of the flip-flop circuit 110, the ratio of the sizes of thetransistor 115 to the transistor 116 is preferably 4:1 or more. This isbecause the amplitude voltage of an output signal of the flip-flopcircuit 110 is increased by the level-shift circuit or the like, so thatthe flip-flop circuit 110 often operates with a low power supplyvoltage.

Alternatively, when the level-shift circuit or the like is not connectedto the output terminal OUT of the flip-flop circuit 110, the ratio ofthe sizes of the transistor 115 to the transistor 116 is preferably 10:1or more. This is because the output signal of the flip-flop circuit 110is applied to some kind of operation without being level shifted, sothat the flip-flop circuit 110 often operates with a high power supplyvoltage.

Note that each of the power supply potentials and potentials of thecontrol signals may be any potential as long as it can control on/off ofa target transistor.

For example, the power supply potential VSS may be a potential lowerthan an L-level potential of a control signal. This is because thepotential of the node N3 is Vn31 (VSS+|Vth116|+|Vth118|), so that Vn31which is the potential of the node N3 becomes lower when the powersupply potential VSS becomes lower. Accordingly, the transistor 113 andthe transistor 114 can be surely turned on even when the thresholdvoltages of the transistor 113 and the transistor 114 become lower dueto the characteristic deterioration of the transistor 113 and thetransistor 114.

In addition, the power supply potential VSS may be a potential higherthan the L-level potential of the control signal as long as it cancontrol on/off of each transistor.

Note that the capacitor 119 is not necessarily provided when gatecapacitance (parasitic capacitance) between the gate terminal and thesecond terminal of the transistor 112 is sufficiently large.

For example, the capacitor 119 is not necessary connected as in aflip-flop circuit 130 in FIG. 13. Accordingly, since the number ofelements in the flip-flop circuit 130 is one less than the number ofelements in the flip-flop circuit 110, each element can be arranged inhigh density in the flip-flop circuit 130.

In addition, as another example, a capacitor may be formed by using atransistor 161 as in a flip-flop circuit 160 in FIG. 16. This is becausethe gate capacitance of the transistor 161 sufficiently functions as acapacitor when the transistor 161 is on.

It is to be noted that since the transistor 161 is on in the period T1and the period T2 (at the time of performing the bootstrap operation), achannel region is formed in the transistor 161 so that the transistor161 functions as a capacitor. On the other hand, since the transistor161 is off in the period T3 and the period T4 (at the time of notperforming the bootstrap operation), a channel region is not formed inthe transistor 161 so that the transistor 161 does not function as acapacitor or functions as a small capacitor.

Here, by forming the capacitor by using the transistor 161 as in theflip-flop circuit 160 in FIG. 16 which is described above, thetransistor 161 functions as the capacitor only when needed (in theperiod T1 and the period T2), and the transistor 161 does not functionas the capacitor when not needed (in the period T3 and the period T4).Therefore, the flip-flop circuit 160 hardly malfunctions due to changesin the potentials of the node N1 and the output terminal OUT.

Note that the transistor 161 has the same polarity as that of thetransistor 112.

It is also to be noted that the first terminal of the transistor 111 maybe connected anywhere in the period T1 and the period T2 as long as itcan make the node N1 into a floating state.

For example, the first terminal of the transistor 111 may be connectedto the input terminal IN1 as in a flip-flop circuit 140 in FIG. 14. Thisis because the node N1 can be made into a floating state in the periodT1 and the period T2 even when the first terminal of the transistor 111is connected to the input terminal IN1.

Note that in the flip-flop circuit 110 in FIG. 11, noise is generated inthe first power supply by parasitic capacitance between the firstterminal and the gate terminal of the transistor 111 when the potentialof the input terminal IN1 is changed. In addition, when a current issupplied from the first power supply to the node N1 by on/off of thetransistor 111, noise is generated in the first power supply by avoltage drop due to the current. Such noise is generated by changes inthe potential of the input terminal IN1.

Here, by connecting as in the flip-flop circuit 140 in FIG. 14 which isdescribed above, the above-described noise can be suppressed. Inaddition, by suppressing the noise in the first power supply, anothercircuit using the first power supply can operate stably.

It is to be noted that another circuit using the first power supplycorresponds to an inverter circuit, a level-shift circuit, a latchcircuit, a PWC circuit, or the like which is connected to the outputterminal OUT of the flip-flop circuit 140.

Note also that any element can be used as the transistor 116 as long asit can form an inverter circuit with the transistor 115. The transistor116 does not necessarily have rectifying properties; any element can beused as long as a voltage is generated in the element when a current issupplied thereto.

For example, a resistor 151 may be connected as a substitute for thetransistor 116 as in a flip-flop circuit 150 in FIG. 15. This is becausean inverter circuit can be formed by using the resistor 151 and thetransistor 115 even when the resistor 151 is connected as a substitutefor the transistor 116.

Note that when the transistor 115 is off, the potential of the node N2becomes VSS which is the same potential as that of the first powersupply. In addition, the potential of the node N3 at this time becomes avalue which is the sum of the power supply potential VSS and theabsolute value of the threshold voltage Vth118 of the transistor 118(VSS+|Vth118|).

Here, by using the resistor 151 as a substitute for the transistor 116as in the flip-flop circuit 150 in FIG. 15 which is described above, thepotential of the node N2 becomes VSS and the potential of the node N3only becomes higher than VSS by the threshold voltage Vth118 of thetransistor 118 even when the threshold voltage of each transistorbecomes higher due to characteristic deterioration, and thus, thetransistor 113 and the transistor 114 can be easily turned on.

It is to be noted that although a control signal is supplied to each ofthe input terminal IN1, the input terminal IN2, the input terminal IN3,and the input terminal IN4, the invention is not limited to this.

For example, each of the input terminal IN1 the input terminal IN2, theinput terminal IN3, and the input terminal IN4 may be supplied with thepower supply potential VDD, the power supply potential VSS, or anotherpotential.

It is to be noted that although the first terminal of the transistor 111and the first terminal of the transistor 116 are connected to the firstpower supply, the invention is not limited to this.

For example, the first terminal of the transistor 111 and the firstterminal of the transistor 116 may be connected to different powersupplies, respectively. In that case, a potential of a power supplyconnected to the first terminal of the transistor 116 is preferablyhigher than a potential of a power supply connected to the firstterminal of the transistor 111.

As another example, a control signal may be supplied to each of thefirst terminal of the transistor 111 and the first terminal of thetransistor 116.

It is to be noted that although the first terminal of the transistor113, the first terminal of the transistor 114, and the first terminal ofthe transistor 117 are connected to the second power supply, theinvention is not limited to this.

For example, the first terminal of the transistor 113, the firstterminal of the transistor 114, and the first terminal of the transistor117 may be connected to different power supplies, respectively.

Note that this embodiment mode can be freely implemented in combinationwith any description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor in theshift register circuit of the invention is turned on at regularintervals, so that a power supply potential is supplied to the outputterminal. Therefore, the power supply potential is supplied to theoutput terminal of the shift register circuit through the transistor.Since the transistor is not always on in the non-selection period, thethreshold voltage shift of the transistor can be suppressed. Further,the power supply potential is supplied to the output terminal of theshift register circuit through the transistor at regular intervals.Therefore, the shift register circuit can suppress noise which isgenerated in the output terminal.

Embodiment Mode 2

In this embodiment mode, a configuration of a shift register circuit ofthe invention is described.

FIG. 17 shows one mode of the shift register circuit of the invention. Ashift register circuit shown in FIG. 17 includes a plurality offlip-flop circuits 171, a control signal line 172, a control signal line173, and a control signal line 174.

As shown in the shift register circuit in FIG. 17, the input terminalIN1 in each of the flip-flop circuits 171 is connected to the outputterminal OUT of a flip-flop circuit 171 in the previous stage. Theoutput terminal OUT is connected to the input terminal IN1 of aflip-flop circuit 171 in the next stage, the input terminal IN4 of aflip-flop circuit 171 in the previous stage, and the output terminalSRout of the shift register circuit. Note that the input terminal IN1 ofa flip-flop circuit 171 in a first stage is connected to the controlsignal line 172. In addition, the input terminal IN4 of a flip-flopcircuit 171 in the last stage is connected to a power supply. Inflip-flop circuits 171 in odd-numbered stages, input terminals IN2 areconnected to the control signal line 173 and input terminals IN3 areconnected to the control signal line 174. On the other hand, inflip-flop circuits 171 in even-numbered stages, input terminals IN2 areconnected to the control signal line 174 and input terminals IN3 areconnected to the control signal line 173.

Note that flip-flop circuits which are similar to those shown inEmbodiment Mode 1 can be used as the flip-flop circuits 171.

In addition, input terminals IN1 to IN4 and output terminals OUT whichare similar to those shown in Embodiment Mode 1 can be used as the inputterminals IN1 to IN4 and the output terminals OUT of the flip-flopcircuits 171.

Further, an output terminal SRout in a first stage of the shift registercircuit of the invention is denoted by SRout1; an output terminal SRoutin a second stage of the shift register circuit of the invention isdenoted by SRout2; an output terminal SRout in a third stage of theshift register circuit of the invention is denoted by SRout3; an outputterminal SRout in a fourth stage of the shift register circuit of theinvention is denoted by SRout4; an output terminal SRout in an n-thstage of the shift register circuit of the invention is denoted bySRoutn.

In addition, in the flip-flop circuits 171, a power supply and a powersupply line are not illustrated for convenience. The first power supplyand the second power supply which are described in Embodiment Mode 1 canbe used as the power supply and the power supply line. Accordingly, thepotential difference (VDD−VSS) between the power supply potential VDD ofthe first power supply and the power supply potential VSS of the secondpower supply corresponds to a power supply voltage of the flip-flopcircuit 171.

Further, control signals SSP, CK, and CKB are supplied to the controlsignal line 172 to the control signal line 174, respectively. Inaddition, output signals of the flip-flop circuits 171 in the firststage to fourth stage and the n-th stage are supplied to the outputterminals SRout1 to SRout4 and the output terminal SRoutn of the shiftregister circuit, respectively.

Next, operations of the shift register circuit shown in FIG. 17 aredescribed with reference to a timing chart shown in FIG. 18. FIG. 18 isa timing chart of the control signals SSP, CK, and CKB supplied to thecontrol signal lines 172 to 174, respectively, and the output signals ofthe output terminals SRout1 to SRout4 and the output terminal SRoutn. Inaddition, the timing chart shown in FIG. 18 is divided into a period T0to a period T5, a period Tn, and a period Tn+1 for convenience.

It is to be noted that FIG. 18 is a timing chart in the case of usingn-channel transistors as transistors. That is, FIG. 18 is a timing chartin the case of using the flip-flop circuits shown in FIG. 1 and FIGS. 7to 10 as the flip-flop circuits 171.

Note that in the timing chart shown in FIG. 18, each of a control signaland the output signal is a digital signal having two values similar toEmbodiment Mode 1.

The operations of the shift register circuit shown in FIG. 17 aredescribed with reference to FIG. 18.

First, an operation of the shift register circuit in the period T0 isdescribed. In the period T0, the control signal SSP is at an H level;the control signal CK is at an L level; and the control signal CKB is atan H level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1becomes an H level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 becomes an Llevel. Thus, the output terminal OUT becomes an L level. This state isthe same as that of the timing chart shown in FIG. 2 in the period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage, the input terminal IN1 becomes an L level; the inputterminal IN2 becomes an L level; the input terminal IN3 becomes an Hlevel; and the input terminal IN4 becomes an L level. Thus, the outputterminal OUT becomes an L level. This state is the same as that of thetiming chart shown in FIG. 2 in the period T3.

In the flip-flop circuits 171 in the even-numbered stages, the inputterminal IN1 becomes an L level; the input terminal IN2 becomes an Hlevel; the input terminal IN3 becomes an L level; and the input terminalIN4 becomes an L level. Thus, the output terminal OUT becomes an Llevel. This state is the same as that of the timing chart shown in FIG.2 in the period T4.

In this manner, all the output terminals SRout of the shift registercircuit are at an L level.

Next, an operation of the shift register circuit in the period T1 isdescribed. In the period T1, the control signal SSP is at an L level;the control signal CK is at an H level; and the control signal CKB is atan L level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1becomes an L level; the input terminal IN2 becomes an H level; the inputterminal IN3 becomes an L level; and the input terminal IN4 is unchangedat an L level. Thus, the output terminal OUT becomes an H level. Thisstate is the same as that of the timing chart shown in FIG. 2 in theperiod T2.

In the flip-flop circuit 171 in the second stage, the input terminal IN1becomes an H level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 is unchangedat an L level. Thus, the output terminal OUT is unchanged at an L level.This state is the same as that of the timing chart shown in FIG. 2 inthe period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage, the input terminal IN1 is unchanged at an L level; theinput terminal IN2 becomes an H level; the input terminal IN3 becomes anL level; and the input terminal IN4 is unchanged at an L level. Thus,the output terminal OUT is unchanged at an L level. This state is thesame as that of the timing chart shown in FIG. 2 in the period T4.

In the flip-flop circuits 171 in the even-numbered stages except for thesecond stage, the input terminal IN1 is unchanged at an L level; theinput terminal IN2 becomes an L level; the input terminal IN3 becomes anH level; and the input terminal IN4 is unchanged at an L level. Thus,the output terminal OUT is unchanged at an L level. This state is thesame as that of the timing chart shown in FIG. 2 in the period T3.

In this manner, the output terminal SRout1 of the shift register circuitbecomes an H level, and other output terminals SRout are unchanged at anL level.

Next, an operation of the shift register circuit in the period T2 isdescribed. In the period T2, the control signal SSP becomes an L level;the control signal CK becomes an L level; and the control signal CKBbecomes an H level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1is unchanged at an L level; the input terminal IN2 becomes an L level;the input terminal IN3 becomes an L level; and the input terminal IN4becomes an H level. Thus, the output terminal OUT becomes an L level.This state is the same as that of the timing chart shown in FIG. 2 inthe period T3.

In the flip-flop circuit 171 in the second stage, the input terminal IN1becomes an L level; the input terminal IN2 becomes an H level; the inputterminal IN3 becomes an L level; and the input terminal IN4 is unchangedat an L level. Thus, the output terminal OUT becomes an H level. Thisstate is the same as that of the timing chart shown in FIG. 2 in theperiod T2.

In the flip-flop circuit 171 in the third stage, the input terminal IN1becomes an H level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 is unchangedat an L level. Thus, the output terminal OUT is unchanged at an L level.This state is the same as that of the timing chart shown in FIG. 2 inthe period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage and the third stage, the input terminal IN1 is unchanged atan L level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 is unchangedat an L level. Thus, the output terminal OUT is unchanged at an L level.This state is the same as that of the timing chart shown in FIG. 2 inthe period T3.

In the flip-flop circuits 171 in the even-numbered stages except for thesecond stage, the input terminal IN1 is unchanged at an L level; theinput terminal IN2 becomes an H level; the input terminal IN3 becomes anL level; and the input terminal IN4 is unchanged at an L level. Thus,the output terminal OUT is unchanged at an L level. This state is thesame as that of the timing chart shown in FIG. 2 in the period T4.

In this manner, the output terminal SRout1 of the shift register circuitbecomes an L level; the output terminal SRout2 becomes an H level; andother output terminals SRout are unchanged at an L level.

Similarly in the later periods, the output terminal SRout3 of the shiftregister circuit becomes an H level in the period T3; the outputterminal SRout4 of the shift register circuit becomes an H level in theperiod T4; the output terminal SRout5 of the shift register circuit inthe fifth stage becomes an H level in the period T5; and the outputterminal SRoutn of the shift register circuit in the n-th stage becomesan H level in the period Tn. In this manner, the output terminals of theshift register circuit sequentially become an H level only for oneperiod. In addition, one period corresponds to a half period of thecontrol signal CK or the control signal CKB.

By the above-described operations, the output terminals SRout of theshift register circuits shown in FIG. 17 can be set at an H level onestage by one stage. In addition, by using the flip-flop circuits shownin Embodiment Mode 1 as the flip-flop circuits 171, the flip-flopcircuits shown in FIG. 17 hardly malfunction due to characteristicdeterioration of the transistors so that noise of the output signals isreduced.

Although FIG. 18 shows the timing chart in the case where thetransistors of the flip-flop circuits 171 are n-channel transistors,FIG. 19 shows a timing chart in the case where transistors of theflip-flop circuits 171 are p-channel transistors. That is, FIG. 19 is atiming chart in the case of using the flip-flop circuits shown in FIG.11 and FIGS. 13 to 16 as the flip-flop circuits 171.

Next, operations of the shift register circuit shown in FIG. 17 aredescribed with reference to a timing chart shown in FIG. 19. FIG. 19 isa timing chart of the control signals SSP, CK, and CKB supplied to thecontrol signal lines 172 to 174, respectively, and the output signals ofthe output terminals SRout1 to SRout4 and the output terminal SRoutnshown in FIG. 17. In addition, the timing chart shown in FIG. 19 isdivided into a period T0 to a period T5, a period Tn, and a period Tn+1for convenience. Note that with respect to the timing of the controlsignals and the output signals, an H level and an L level are invertedfrom those in the case where the flip-flop circuit 171 is formed byusing all n-channel transistors (FIG. 18).

Note that in the timing chart shown in FIG. 19, each of the controlsignal and the output signal is a digital signal having two valuessimilar to Embodiment Mode 1.

The operations of the shift register circuit shown in FIG. 17 aredescribed with reference to FIG. 19.

First, an operation of the shift register circuit in the period T0 isdescribed. In the period T0, the control signal SSP is at an L level;the control signal CK is at an H level; and the control signal CKB is atan L level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1becomes an L level; the input terminal IN2 becomes an H level; the inputterminal IN3 becomes an L level; and the input terminal IN4 becomes an Hlevel. Thus, the output terminal OUT becomes an H level. This state isthe same as that of the timing chart shown in FIG. 12 in the period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage, the input terminal IN1 becomes an L level; the inputterminal IN2 becomes an H level; the input terminal IN3 becomes an Llevel; and the input terminal IN4 becomes an H level. Thus, the outputterminal OUT becomes an H level. This state is the same as that of thetiming chart shown in FIG. 12 in the period T3.

In the flip-flop circuits 171 in the even-numbered stages, the inputterminal IN1 becomes an H level; the input terminal IN2 becomes an Llevel; the input terminal IN3 becomes an H level; and the input terminalIN4 becomes an H level. Thus, the output terminal OUT becomes an Hlevel. This state is the same as that of the timing chart shown in FIG.12 in the period T4.

In this manner, all the output terminals SRout of the shift registercircuit are at an H level.

Next, an operation of the shift register circuit in the period T1 isdescribed. In the period T1, the control signal SSP is at an H level;the control signal CK is at an L level; and the control signal CKB is atan H level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1becomes an H level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 is unchangedat an H level. Thus, the output terminal OUT becomes an L level. Thisstate is the same as that of the timing chart shown in FIG. 12 in theperiod T2.

In the flip-flop circuit 171 in the second stage, the input terminal IN1becomes an L level; the input terminal IN2 becomes an H level; the inputterminal IN3 becomes an L level; and the input terminal IN4 is unchangedat an H level. Thus, the output terminal OUT is unchanged at an H level.This state is the same as that of the timing chart shown in FIG. 12 inthe period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage, the input terminal IN1 is unchanged at an H level; theinput terminal IN2 becomes an L level; the input terminal IN3 becomes anH level; and the input terminal IN4 is unchanged at an H level. Thus,the output terminal OUT is unchanged at an H level. This state is thesame as that of the timing chart shown in FIG. 12 in the period T4.

In the flip-flop circuits 171 in the even-numbered stages except for thesecond stage, the input terminal IN1 is unchanged at an H level; theinput terminal IN2 becomes an H level; the input terminal IN3 becomes anL level; and the input terminal IN4 is unchanged at an H level. Thus,the output terminal OUT is unchanged at an H level. This state is thesame as that of the timing chart shown in FIG. 12 in the period T3.

In this manner, the output terminal SRout1 of the shift register circuitbecomes an L level, and other output terminals SRout are unchanged at anH level.

Next, an operation of the shift register circuit in the period T2 isdescribed. In the period T2, the control signal SSP is at an H level;the control signal CK is at an H level; and the control signal CKB is atan L level.

In the flip-flop circuit 171 in the first stage, the input terminal IN1is unchanged at an H level; the input terminal IN2 becomes an H level;the input terminal IN3 becomes an L level; and the input terminal IN4becomes an L level. Thus, the output terminal OUT becomes an H level.This state is the same as that of the timing chart shown in FIG. 12 inthe period T3.

In the flip-flop circuit 171 in the second stage, the input terminal IN1becomes an H level; the input terminal IN2 becomes an L level; the inputterminal IN3 becomes an H level; and the input terminal IN4 is unchangedat an H level. Thus, the output terminal OUT becomes an L level. Thisstate is the same as that of the timing chart shown in FIG. 12 in theperiod T2.

In the flip-flop circuit 171 in the third stage, the input terminal IN1becomes an L level; the input terminal IN2 becomes an H level; the inputterminal IN3 becomes an L level; and the input terminal IN4 is unchangedat an H level. Thus, the output terminal OUT is unchanged at an H level.This state is the same as that of the timing chart shown in FIG. 12 inthe period T1.

In the flip-flop circuits 171 in the odd-numbered stages except for thefirst stage and third stage, the input terminal IN1 is unchanged at an Hlevel; the input terminal IN2 becomes an H level; the input terminal IN3becomes an L level; and the input terminal IN4 is unchanged at an Hlevel. Thus, the output terminal OUT is unchanged at an H level. Thisstate is the same as that of the timing chart shown in FIG. 12 in theperiod T3.

In the flip-flop circuits 171 in the even-numbered stages except for thesecond stage, the input terminal IN1 is unchanged at an H level; theinput terminal IN2 becomes an L level; the input terminal IN3 becomes anH level; and the input terminal IN4 is unchanged at an H level. Thus,the output terminal OUT is unchanged at an H level. This state is thesame as that of the timing chart shown in FIG. 12 in the period T4.

In this manner, the output terminal SRout1 of the shift register circuitbecomes an H level; the output terminal SRout2 becomes an L level; andother output terminals SRout are unchanged at an H level.

Similarly in the later periods, the output terminal SRout3 of the shiftregister circuit becomes an L level in the period T3; the outputterminal SRout4 of the shift register circuit becomes an L level in theperiod T4; the output terminal SRout5 of the shift register circuit inthe fifth stage becomes an L level in the period T5; and the outputterminal SRoutn of the shift register circuit in the n-th stage becomesan L level in the period Tn. In this manner, the output terminals of theshift register circuit sequentially become an L level only for oneperiod. In addition, one period corresponds to a half period of thecontrol signal CK or the control signal CKB.

By the above-described operations, the output terminal SRout of theshift register circuit shown in FIG. 17 can be set at an L level onestage by one stage. In addition, by using the flip-flop circuits shownin Embodiment Mode 1 as the flip-flop circuits 171, the flip-flopcircuits shown in FIG. 17 hardly malfunction due to characteristicdeterioration of the transistors so that noise of the output signals isreduced.

It is to be noted that the flip-flop circuits 171 may be any flip-flopcircuits as long as they can supply selection signals to the outputterminals SRout of the shift register circuit sequentially from thefirst stage.

Note that the output terminals OUT of the flip-flop circuits 171 may beconnected to the output terminals SRout of the shift register circuitthrough various elements and circuits. Various elements and circuitscorrespond to a logic circuit such as an inverter circuit, a buffercircuit, a NAND circuit, a NOR circuit, a tristate buffer circuit, or aPWC circuit, and a switch, a resistor, a capacitor, another element, orthe like. In addition, by combining with these elements or circuits,various circuits can be formed.

It is to be noted that although a control signal is supplied to each ofthe control signal lines 172 to 174, the invention is not limited tothis.

For example, each of the control signal lines 172 to 174 may be suppliedwith the power supply potential VDD, the power supply potential VSS, oranother potential.

It is to be noted that although the control signal CK is supplied to thecontrol signal line 173 and the control signal CKB is supplied to thecontrol signal line 174, the invention is not limited to this.

For example, the control signal CK may be supplied to the control signalline 173 and an inverted signal of the control signal CK may be suppliedto the control signal line 174 through an inverter circuit.Alternatively, an inverted signal of the control signal CKB may besupplied to the control signal line 173 through an inverter circuit andthe control signal CKB may be supplied to the control signal line 174.Note that this inverter circuit is preferably formed over the samesubstrate as the shift register circuit.

It is to be noted that although the input terminal IN4 of the flip-flopcircuit 171 in the last stage is connected to the power supply, theinvention is not limited to this.

For example, the input terminal IN4 of the flip-flop circuit 171 in thelast stage may be connected to any one of the control signal lines 172to 174, to another control signal line, or to the output terminal OUT ofthe flip-flop circuit 171 in another stage.

Note that this embodiment mode can be freely implemented in combinationwith any description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor in theshift register circuit of the invention is turned on at regularintervals, so that a power supply potential is supplied to the outputterminal. Therefore, the power supply potential is supplied to theoutput terminal of the shift register circuit through the transistor.Since the transistor is not always on in the non-selection period, thethreshold voltage shift of the transistor can be suppressed. Further,the power supply potential is supplied to the output terminal of theshift register circuit through the transistor at regular intervals.Therefore, the shift register circuit can suppress noise which isgenerated in the output terminal.

Embodiment Mode 3

In this embodiment mode, a structure example in the case of using theflip-flop circuit described in Embodiment Mode 1, the shift registercircuit described in Embodiment Mode 2, and the like as a part of adriver circuit is described.

A structure example of a driver circuit which can be applied to a gatedriver is described with reference to FIGS. 20 to 27. Note that drivercircuits in FIGS. 20 to 27 can be applied not only to gate drivers butalso to any circuit structures.

FIG. 20 shows one mode of a gate driver of the invention. The gatedriver of the invention includes a shift register circuit 200 and abuffer circuit 201.

As shown in the gate driver in FIG. 20, an output terminal SRout of theshift register circuit 200 is connected to an output terminal GDout ofthe gate driver through the buffer circuit 201.

Note that the shift register circuit 200 is similar to that described inEmbodiment Mode 2.

In addition, output terminals SRout1 to SRout4 and an output terminalSRoutn of the shift register circuit 200 are the same as those describedin Embodiment Mode 2.

Further, an output terminal GDout in a first stage of the gate driver ofthe invention is denoted by GDout1; an output terminal GDout in a secondstage of the gate driver of the invention is denoted by GDout2; anoutput terminal GDout in a third stage of the gate driver of theinvention is denoted by GDout3; and an output terminal GDout in an n-thstage of the gate driver of the invention is denoted by GDoutn.

In addition, the buffer circuit 201 includes a logic circuit such as aninverter circuit, a buffer circuit, a NAND circuit, a NOR circuit, atristate buffer circuit, or a PWC circuit, a switch, a resistor, acapacitor, another element, or the like. In addition, by combining withthese elements and circuits, various circuits can be formed.

Further, in the gate driver in FIG. 20, a power supply line and acontrol signal line are not illustrated for convenience.

Furthermore, in the case where the shift register circuit 200 is formedby using an n-channel transistor, the buffer circuit 201 is preferablyformed by using an n-channel transistor as well. In the case where theshift register circuit 200 is formed by using a p-channel transistor,the buffer circuit 201 is preferably formed by using a p-channeltransistor as well.

In addition, in the case where the shift register circuit 200 is formedby using an n-channel transistor, an output signal of the shift registercircuit 200 is the same as that of the timing chart in FIG. 18. In thecase where the shift register circuit 200 is formed by using a p-channeltransistor, an output signal of the shift register circuit 200 is thesame as that of the timing chart in FIG. 19.

Here, a specific structure example of the buffer circuit 201 isdescribed. FIGS. 21 to 27 show structure examples of the gate driverincluding the buffer circuit. Note that a structure of the buffercircuit 201 is not limited to structures in FIGS. 21 to 27.

FIG. 21 specifically shows one mode of the gate driver including thebuffer circuit of the invention. A gate driver in FIG. 21 includes theshift register circuit 200 and a buffer circuit 210. The buffer circuit210 includes an inverter circuit 211A in the first stage and an invertercircuit 211B in the second stage.

As shown in the gate driver in FIG. 21, the output terminal SRout of theshift register circuit 200 is connected to the output terminal GDout ofthe gate driver through the buffer circuit 210.

Connection relations in the buffer circuit 210 are described. The inputterminal IN of the inverter circuit 211A is connected to the outputterminal SRout of the shift register circuit 200, and the outputterminal OUT of the inverter circuit 211A is connected to the inputterminal IN of the inverter circuit 211B. The output terminal OUT of theinverter circuit 211B is connected to the output terminal GDout of thegate driver. That is, in the buffer circuit 210, two inverter circuits211A and 211B are connected in series for each output terminal SRout ofthe shift register circuit 200 in each stage.

Operations of the gate driver in FIG. 21 in the case where the outputterminal SRout is at an H level and in the case where the outputterminal SRout is at an L level are described, respectively.

First, the case where the output terminal SRout is at an H level isdescribed. Since the output terminal SRout is connected to the outputterminal GDout through the two inverter circuits 211A and 211B, theoutput terminal GDout becomes to be at an H level.

Next, the case where the output terminal SRout is at an L level isdescribed. Since the output terminal SRout is connected to the outputterminal GDout through the two inverter circuits 211A and 211B, theoutput terminal GDout becomes to be at an L level.

By the above-described operations, the output terminal GDout becomes tobe at an H level when the output terminal SRout becomes to be at an Hlevel. In addition, the output terminal GDout becomes to be at an Llevel when the output terminal SRout becomes to be at an L level.

In addition, since the inverter circuits 211A and 211B have rectifyingproperties, adverse effect of noise in the output terminal SRout on theoutput terminal GDout of the gate driver can be suppressed.

It is to be noted that although the two inverter circuits 211A and 211Bare connected in series in the buffer circuit 210, a plurality ofinverter circuits 211 may be connected in series. For example, in thecase where an odd number of inverter circuits 211 are connected inseries, the output terminal GDout becomes to be at the opposite level tothat of the output terminal SRout. In the case where an even number ofinverter circuits 211 are connected in series, the output terminal GDoutbecomes to be at the same level as that of the output terminal SRout.

It is to be noted that although the two inverter circuits 211A and 211Bare connected in series in the buffer circuit 210, a plurality ofinverter circuits 211 may be connected in parallel as well. This reducescurrent density in the inverter circuits 211A and 211B, so thatcharacteristic deterioration of elements forming the inverter circuits211A and 211B can be suppressed.

FIG. 22 specifically shows another mode of the gate driver including thebuffer circuit of the invention. A gate driver in FIG. 22 includes theshift register circuit 200, a buffer circuit 220, and a control signalline 222. The buffer circuit 220 includes a NAND circuit 221.

As shown in the gate driver in FIG. 22, the output terminal SRout of theshift register circuit 200 is connected to the output terminal GDout ofthe gate driver through the buffer circuit 220.

Connection relations in the buffer circuit 220 are described. The inputterminal IN1 of the NAND circuit 221 is connected to the control signalline 222; the input terminal IN2 of the NAND circuit 221 is connected tothe output terminal SRout of the shift register circuit 200; and theoutput terminal OUT of the NAND circuit 221 is connected to the outputterminal GDout of the gate driver.

In addition, an enable signal En is supplied to the control signal line222. The enable signal En is a digital signal.

Operations of the gate driver in FIG. 22 in the cases where the controlsignal line 222 is at an H level and is at an L level, and in the caseswhere the output terminal SRout is at an H level and is at an L levelare described, respectively.

First, the case where the control signal line 222 is at an H level andthe output terminal SRout at an H level is described. The input terminalIN1 of the NAND circuit 221 becomes to be at an H level and the inputterminal IN2 of the NAND circuit 221 becomes to be at an H level.Accordingly, since the output terminal OUT of the NAND circuit 221becomes to be at an L level, the output terminal GDout of the gatedriver becomes to be at an L level.

Next, the case where the control signal line 222 is at an H level andthe output terminal SRout is at an L level is described. The inputterminal IN1 of the NAND circuit 221 becomes to be at an H level and theinput terminal IN2 of the NAND circuit 221 becomes to be at an L level.Accordingly, since the output terminal OUT of the NAND circuit 221becomes to be at an H level, the output terminal GDout of the gatedriver becomes to be at an H level.

Next, the case where the control signal line 222 is at an L level andthe output terminal SRout is at an H level is described. The inputterminal IN1 of the NAND circuit 221 becomes to be at an L level and theinput terminal IN2 of the NAND circuit 221 becomes to be at an H level.Accordingly, since the output terminal OUT of the NAND circuit 221becomes to be at an H level, the output terminal GDout of the gatedriver becomes to be at an H level.

Next, the case where the control signal line 222 is at an L level andthe output terminal SRout is at an L level is described. The inputterminal IN1 of the NAND circuit 221 becomes to be at an L level and theinput terminal IN2 of the NAND circuit 221 becomes to be at an L level.Accordingly, since the output terminal OUT of the NAND circuit 221becomes to be at an H level, the output terminal GDout of the gatedriver becomes to be at an H level.

By the above-described operations, when the control signal line 222 isat an H level, the output terminal GDout of the gate driver becomes tobe at an L level when the output terminal SRout is at an H level,whereas the output terminal GDout of the gate driver becomes to be at anH level when the output terminal SRout is at an L level. When thecontrol signal line 222 is at an L level, the output terminal GDout ofthe gate driver becomes to be at an H level regardless of a potential ofthe output terminal SRout.

An output signal of the gate driver can be changed arbitrarily by theenable signal En in this manner. In the gate driver in FIG. 22,so-called pulse width control (PWC) can be performed.

Here, the pulse width control is performed by utilizing that the outputterminal GDout becomes to be at an H level when the enable signal En isat an L level regardless of the potential of the output terminal SRout.That is, even when the output signal of the shift register circuit 200has certain L level pulse width (period), the output signal can beshortened by making the enable signal En at an L level.

Note that although the NAND circuit 221 has two input terminals, theNAND circuit 221 may have any number of input terminals as long as theoutput signal of the shift register circuit 200 is supplied to any oneof the input terminals. When the NAND circuit 221 has a plurality ofinput terminals, the buffer circuit 220 can control the output signal ofthe gate driver more exactly.

It is to be noted that the output terminal SRout may be connected to theinput terminal IN2 of the NAND circuit 221 through the inverter circuit211 as in a buffer circuit 240 in FIG. 24. In this case, when thecontrol signal line 222 is at an H level, the output terminal GDout ofthe gate driver becomes to be at an H level when the output terminalSRout is at an H level, whereas the output terminal GDout of the gatedriver becomes to be at an H level when the output terminal SRout is atan L level. When the control signal line 222 is at an L level, theoutput terminal GDout of the gate driver becomes to be at an H levelregardless of the potential of the output terminal SRout.

It is to be noted that the output terminal OUT of the NAND circuit 221may be connected to the output terminal GDout of the gate drive throughthe inverter circuit 211 as in a buffer circuit 260 in FIG. 26. In thiscase, when the control signal line 222 is at an H level, the outputterminal GDout of the gate driver becomes to be at an L level when theoutput terminal SRout is at an H level, and the output terminal GDout ofthe gate driver becomes to be at an L level when the output terminalSRout is at an L level. When the control signal line 222 is at an Llevel, the output terminal GDout of the gate driver becomes to be at anL level regardless of the potential of the output terminal SRout.

It is to be noted that although the enable signal En is supplied to thecontrol signal line 222, the invention is not limited to this.

For example, a different control signal may be supplied to the controlsignal line 222.

As another example, a power supply may be supplied to the control signalline 222.

FIG. 23 specifically shows another mode of the gate driver including thebuffer circuit of the invention. A gate driver in FIG. 23 includes theshift register circuit 200, a buffer circuit 230, and the control signalline 222. The buffer circuit 230 includes a NOR circuit 231.

As shown in the gate driver in FIG. 23, the output terminal SRout of theshift register circuit 200 is connected to the output terminal GDout ofthe gate driver through the buffer circuit 230.

Connection relations in the buffer circuit 230 are described. The inputterminal IN1 of the NOR circuit 231 is connected to the control signalline 222; the input terminal IN2 of the NOR circuit 231 is connected tothe output terminal SRout of the shift register circuit 200; and theoutput terminal OUT of the NOR circuit 231 is connected to the outputterminal GDout of the gate driver.

In addition, the enable signal En is supplied to the control signal line222.

Operations of the gate driver in FIG. 23 in the cases where the controlsignal line 222 is at an H level and is at an L level, and in the caseswhere the output terminal SRout of the shift register circuit 200 is atan H level and is at an L level are described, respectively.

First, the case where the control signal line 222 is at an H level andthe output terminal SRout of the shift register circuit 200 is at an Hlevel is described. The input terminal IN1 of the NOR circuit 231becomes to be at an H level and the input terminal IN2 of the NORcircuit 231 becomes to be at an H level. Accordingly, since the outputterminal OUT of the NOR circuit 231 becomes to be at an L level, theoutput terminal GDout of the gate driver becomes to be at an L level.

Next, the case where the control signal line 222 is at an H level andthe output terminal SRout of the shift register circuit 200 is at an Llevel is described. The input terminal IN1 of the NOR circuit 231becomes to be at an H level and the input terminal IN2 of the NORcircuit 231 becomes to be at an L level. Accordingly, since the outputterminal OUT of the NOR circuit 231 becomes to be at an L level, theoutput terminal GDout of the gate driver becomes to be at an L level.

Next, the case where the control signal line 222 is at an L level andthe output terminal SRout of the shift register circuit 200 is at an Hlevel is described. The input terminal IN1 of the NOR circuit 231becomes to be at an L level and the input terminal IN2 of the NORcircuit 231 becomes to be at an H level. Accordingly, since the outputterminal OUT of the NOR circuit 231 becomes to be at an L level, theoutput terminal GDout of the gate driver becomes to be at an L level.

Next, the case where the control signal line 222 is at an L level andthe output terminal SRout of the shift register circuit 200 is at an Llevel is described. The input terminal IN1 of the NOR circuit 231becomes to be at an L level and the input terminal IN2 of the NORcircuit 231 becomes to be at an L level. Accordingly, since the outputterminal OUT of the NOR circuit 231 becomes to be at an H level, theoutput terminal GDout of the gate driver becomes to be at an H level.

By the above-described operations, when the control signal line 222 isat an H level, the output terminal GDout of the gate driver becomes tobe at an L level regardless of the potential of the output terminalSRout. When the control signal line 222 is at an L level, the outputterminal GDout of the gate driver becomes to be at an L level when theoutput terminal SRout is at an H level, whereas the output terminalGDout of the gate driver becomes to be at an H level when the outputterminal SRout is at an L level

The output terminal GDout of the gate driver can be changed arbitrarilyby the enable signal En in this manner. In the gate driver in FIG. 23,so-called pulse width control (PWC) can be performed.

Here, the pulse width control is performed by utilizing that the outputterminal GDout becomes to be at an L level when the enable signal En isat an H level regardless of the potential of the output terminal SRout.That is, even when the output signal of the shift register circuit 200has certain H level pulse width (period), the output signal can beshortened by making the enable signal En at an H level.

Note that although the NOR circuit 231 has two input terminals, the NORcircuit 231 may have any number of input terminals as long as the outputsignal of the shift register circuit 200 is supplied to any one of theinput terminals. When the NOR circuit 231 has a plurality of inputterminals, the buffer circuit 230 can control the output signal of thegate driver more correctly.

It is to be noted that the output terminal SRout of the shift registercircuit 200 may be connected to the input terminal IN2 of the NORcircuit 231 through the inverter circuit 211 as in a buffer circuit 250in FIG. 25. In this case, when the control signal line 222 is at an Hlevel, the output terminal GDout of the gate driver becomes to be at anL level regardless of the potential of the output terminal SRout. Whenthe control signal line 222 is at an L level, the output terminal GDoutof the gate driver becomes to be at an H level when the output terminalSRout is at an H level, and the output terminal GDout of the gate driverbecomes to be at an L level when the output terminal SRout is at an Llevel.

It is to be noted that the output terminal OUT of the NOR circuit 231may be connected to the output terminal GDout of the gate drive throughthe inverter circuit 211 as in a buffer circuit 270 in FIG. 27. In thiscase, when the control signal line 222 is at an H level, the outputterminal GDout of the gate driver becomes to be at an H level regardlessof the potential of the output terminal SRout. When the control signalline 222 is at an L level, the output terminal GDout of the gate driverbecomes to be at an H level when the output terminal SRout is at an Hlevel, whereas the output terminal GDout outputs an L-level signal whenthe output terminal SRout is at an L level.

Here, a suture example which can be applied to the inverter circuit 211is described.

FIG. 28 shows one mode of the inverter circuit 211. An inverter circuit280 in FIG. 28 includes a transistor 281 and a transistor 282.

As shown in the inverter circuit 280 in FIG. 28, a first terminal of thetransistor 281 is connected to the second power supply; a secondterminal of the transistor 281 is connected to a second terminal of thetransistor 282 and the output terminal OUT; and a gate terminal of thetransistor 281 is connected to the input terminal IN. A first terminalis connected to the first power supply and a gate terminal of thetransistor 282 is connected to the first power supply.

It is to be noted that the power supply potential VDD is supplied to thefirst power supply and the power supply potential VSS is supplied to thesecond power supply. The potential difference (VDD−VSS) between thepower supply potential VDD of the first power supply and the powersupply potential VSS of the second power supply corresponds to a powersupply voltage of the inverter circuit 280. In addition, the powersupply potential VDD is higher than the power supply potential VSS.

It is to be noted that a digital control signal is supplied to the inputterminal IN. In addition, the output terminal OUT outputs an outputsignal.

In addition, each of the transistor 281 and the transistor 282 is ann-channel transistor.

Operations of the inverter circuit 280 in FIG. 28 in the case where theinput terminal IN is at an H level and in the case where the inputterminal IN is at an L level are described, respectively.

First, the input terminal IN at an H level is described. When the inputterminal IN becomes to be at an H level, the transistor 281 is turnedon. The output terminal OUT is electrically connected to the secondpower supply through the transistor 281 and is electrically connected tothe first power supply through the transistor 282, and thus, thepotential of the output terminal OUT drops. The potential of the outputterminal OUT at this time is determined by an operating point of thetransistor 281 and the transistor 282, so that the output terminal OUTbecomes to be at an L level.

Next, the input terminal IN at an L level is described. When the inputterminal IN becomes to be at an L level, the transistor 281 is turnedoff. The output terminal OUT is electrically connected to the firstpower supply through the transistor 282, and the potential of the outputterminal OUT rises. The potential of the output terminal OUT at thistime becomes a value obtained by subtracting the threshold voltageVth282 of the transistor 282 from the power supply potential VDD(VDD−Vth282), so that the output terminal OUT becomes to be at an Hlevel.

The transistor 282 does not necessarily have rectifying properties; anyelement can be used as long as a voltage is generated in the elementwhen a current is supplied thereto. For example, a resistor 321 may beconnected as a substitute for the transistor 282 as in an invertercircuit 320 in FIG. 32.

Here, functions of the transistor 281 and the transistor 282 aredescribed below.

The transistor 281 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with a potential of the input terminal IN. When the inputterminal IN is at an H level, the transistor 281 has a function ofsupplying the power supply potential VSS to the output terminal OUT.

The transistor 282 has a function as a diode.

FIG. 29 shows another mode of the inverter circuit 211. An invertercircuit 290 shown in FIG. 29 includes a transistor 291, a transistor292, a transistor 293, and a capacitor 294 having two electrodes. Notethat the capacitor 294 is not necessarily provided.

As shown in the inverter circuit 290 in FIG. 29, a first terminal of thetransistor 291 is connected to the second power supply; a secondterminal of the transistor 291 is connected to a second terminal of thetransistor 292, a second electrode of the capacitor 294 and the outputterminal OUT; and a gate terminal of the transistor 291 is connected tothe input terminal IN. A first terminal of the transistor 292 isconnected to the first power supply, and a gate terminal of thetransistor 292 is connected to a second terminal of the transistor 293and a first electrode of the capacitor 294. A first terminal isconnected to the first power supply and a gate terminal of thetransistor 293 is connected to the first power supply.

Note that a first power supply, a second power supply, an input terminalIN, and an output terminal OUT which are the same as those shown in FIG.28 can be used as the first power supply, the second power supply, theinput terminal IN, and the output terminal OUT.

In addition, each of the transistors 291 to 293 is an n-channeltransistor.

Operations of the inverter circuit 290 in FIG. 29 in the case where theinput terminal IN is at an H level and in the case where the inputterminal IN is at an L level are described, respectively.

First, the input terminal IN at an H level is described. When the inputterminal IN becomes to be at an H level, the transistor 291 is turnedon. A potential of the gate terminal of the transistor 292 becomes to beat a potential value obtained by subtracting the threshold voltageVth293 of the transistor 293 from the power supply potential VDD(VDD−Vth293), so that the transistor 292 is on. In addition, the gateterminal of the transistor 292 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 291 and is electricallyconnected to the first power supply through the transistor 292, andthus, the potential of the output terminal OUT drops. The potential ofthe output terminal OUT at this time is determined by an operating pointof the transistor 291 and the transistor 292, so that the outputterminal OUT becomes to be at an L level.

Next, the input terminal IN at an L level is described. When the inputterminal IN becomes to be at an L level, the transistor 291 is turnedoff. The potential of the gate terminal of the transistor 292 becomes tobe at a potential value obtained by subtracting the threshold voltageVth293 of the transistor 293 from the power supply potential VDD(VDD−Vth293), so that the transistor 292 is on. In addition, the gateterminal of the transistor 292 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 292, and the potential of theoutput terminal OUT rises. The potential of the gate terminal of thetransistor 292 rises to a value which is greater than or equal to thesum of the power supply potential VDD and the threshold voltage Vth292of the transistor 292 by the capacitive coupling of the capacitor 294,so that the transistor 292 is continuously kept on. A so-calledbootstrap operation is performed. Accordingly, the potential of theoutput terminal OUT at this time becomes to be at VDD, so that theoutput terminal OUT becomes to be at an H level.

In this manner, the H-level potential of the output terminal OUT can beraised to the power supply potential VDD of the first power supply bythe bootstrap operation in the inverter circuit 290 in FIG. 29.

Note that a circuit structure of the inverter circuit 290 in FIG. 29 isnot limited to the circuit structure in FIG. 29 as long as the bootstrapoperation can be performed when the input terminal IN is at an L level.When the input terminal IN is at an H level, a potential may be suppliedto the gate terminal of the transistor 292.

For example, a transistor 331 may be additionally provided as in aninverter circuit 330 in FIG. 33. This is because the potential of theoutput terminal OUT can be made VSS when the output terminal OUT is atan L level. That is, since the transistor 331 is turned on when theinput terminal IN is at an H level, the gate terminal of the transistor292 becomes to be at an L level. Then, the transistor 292 is turned offso that the output terminal OUT is electrically connected only to thesecond power supply through the transistor 291.

It is to be noted that the transistor 331 is an n-channel transistor.

As another example, the first terminal of the transistor 293 may beconnected to an input terminal INb as in an inverter circuit 360 in FIG.36. This is because the potential of the output terminal OUT can be madeVSS when the out terminal OUT is at an L level. That is, since the inputterminal INb becomes to be at an L level when the input terminal IN isat an H level, the gate terminal of the transistor 292 becomes to be atan L level. Then, the transistor 292 is turned off so that the outputterminal OUT is electrically connected only to the second power supplythrough the transistor 291.

It is to be noted that an inverted signal of a signal of the inputterminal IN is supplied to the input terminal INb. In addition, a methodof producing the signal which is supplied to the input terminal INb isdescribed.

For example, a signal of the input terminal IN may be supplied to theinput terminal INb through an inverter circuit 1241 as shown in FIG.124. In addition, the inverter circuits shown in FIGS. 28 to 35 can beapplied as the inverter circuit 1241.

Note that the inverted signal inputted to the signal of the inputterminal IN is not necessarily supplied to the input terminal INb. Inaddition, the signal which is supplied to the input terminal INb isdescribed below.

For example, when the input terminal IN is connected to the outputterminal SRoutn in the n-th stage, the input terminal INb may beconnected to an output terminal SRoutn−1 in the (n−1)th stage.

As another example, when the input terminal IN is connected to theoutput terminal SRoutn in the n-th stage, the input terminal INb may beconnected to an output terminal SRoutn+1 in the (n+1)th stage.

As another example, when the input terminal IN is connected to theoutput terminal SRoutn in the n-th stage, the input terminal INb may beconnected to the node N2 of the flip-flop circuit in the n-th stage.This is because the potential of the node N2 of the flip-flop circuit isan inverted potential of the potential of the output terminal SRout in anon-selection period, so that the potential of the node N2 in theflip-flop circuit can be utilized as an inverted signal. Accordingly, aninverter circuit for producing an inverted signal is not required bysupplying the potential of the node N2 of the flip-flop circuit to theinput terminal INb of the inverter circuit 360.

As another example, when a control signal (a digital value) is suppliedto the input terminal INb, the inverter circuit in FIG. 36 can operateas a tristate buffer circuit. This is because when the input terminal INbecomes to be at an L level and the input terminal INb becomes to be atan L level, the transistor 291 and the transistor 292 are turned off,and thus, the output terminal OUT is not connected to any powersupplies. Thus, the inverter circuit 360 can have a function as atristate buffer circuit or an inverter circuit.

In this manner, a signal can be supplied to the input terminal INb ofthe inverter circuit 360 by various methods.

An application example of FIG. 29 is further described below.

As another example, the first terminal and the gate terminal of thetransistor 293 may be connected to the input terminal INb, and atransistor 391 may be additionally provided as in an inverter circuit390 in FIG. 39. This is because the potential of the output terminal OUTcan be made VSS when the output terminal OUT is at an L level. That is,when the input terminal INb is at an L level, the gate terminal of thetransistor 292 becomes to be at an L level. Then, the transistor 292 isturned off so that the output terminal OUT is electrically connectedonly to the second power supply through the transistor 291.

Note that any element can be used as the capacitor 294 as long as it hascapacitive properties. For example, a transistor 301, a transistor 341,a transistor 371, and a transistor 401 may be connected as a substitutefor the capacitor 294, respectively, as in an inverter circuit 300 inFIG. 30, in an inverter circuit 340 in FIG. 34, in an inverter circuit370 in FIG. 37, and in an inverter circuit 400 in FIG. 40.

Note that the capacitor 294 is not necessarily provided when acapacitance value between the second terminal and the gate terminal ofthe transistor 292 is sufficiently large. For example, the capacitor 294is not required to be connected as in an inverter circuit 310 in FIG.31, in an inverter circuit 350 in FIG. 35, in an inverter circuit 380 inFIG. 38, and in an inverter Circuit 410 in FIG. 41.

Here, functions of the transistors 291 to 293, the transistor 301, thetransistor 331, the transistor 341, and the capacitor 294 are describedbelow.

The transistor 291 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN. When the inputterminal IN is at an H level, the transistor 291 has a function ofsupplying the power supply potential VSS to the output terminal OUT.

The transistor 292 has a function as a switch which determines whetherto connect the first power supply and the output terminal OUT or not.

The transistor 293 has a function as a diode. In addition, thetransistor 293 has a function of making the gate terminal of thetransistor 292 into a floating state.

The transistor 301 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor292. When the input terminal IN is at an L level, the transistor 301 hasa function of raising the potential of the gate terminal of thetransistor 292.

The transistor 331 has a function as a switch which determines whetherto connect the second power supply and the gate terminal of thetransistor 292 or not in accordance with the potential of the inputterminal IN.

The transistor 341 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor292. When the input terminal IN is at an L level, the transistor 341 hasa function of raising the potential of the gate terminal of thetransistor 292 by a rise of the potential of the output terminal OUT.

The capacitor 294 has a function for changing the potential of the gateterminal of the transistor 292 in accordance with the potential of theoutput terminal OUT. When the input terminal IN is at an L level, thecapacitor 294 has a function of raising the potential of the gateterminal of the transistor 292 by the rise of the potential of theoutput terminal OUT.

In this manner, in the inverter circuits in FIGS. 28 to 41, thepotential of the output terminal OUT can be changed freely by changingthe power supply potential VDD when an H-level signal is output. Thatis, the inverter circuits in FIGS. 28 to 41 can operate not only asinverter circuits, but also as level-shift circuits.

Although the inverter circuits formed by using all n-channel transistorsare described in FIGS. 28 to 41, the inverter circuits may be formed byusing all p-channel transistors as well. Here, inverter circuits formedby using all p-channel transistors are shown in FIGS. 58 to 71.

FIG. 58 shows one mode of the inverter circuit 211. An inverter circuit580 in FIG. 58 includes a transistor 581 and a transistor 582.

As shown in the inverter circuit 580 in FIG. 58, a first terminal of thetransistor 581 is connected to the second power supply; a secondterminal of the transistor 581 is connected to a second terminal of thetransistor 582 and the output terminal OUT; and a gate terminal of thetransistor 581 is connected to the input terminal IN. A first terminalis connected to the first power supply and a gate terminal of thetransistor 582 is connected to the first power supply.

It is to be noted that the power supply potential VSS is supplied to thefirst power supply and the power supply potential VDD is supplied to thesecond power supply. The potential difference (VDD−VSS) between thepower supply potential VSS of the first power supply and the powersupply potential VDD of the second power supply corresponds to a powersupply voltage of the inverter circuit 580. In addition, the powersupply potential VDD is higher than the power supply potential VSS.

It is to be noted that a digital control signal is supplied to the inputterminal IN. In addition, the output terminal OUT outputs an outputsignal.

In addition, each of the transistor 581 and the transistor 582 is ap-channel transistor.

Operations of the inverter circuit 580 in FIG. 58 in the case where theinput terminal IN is at an H level and in the case where the inputterminal IN is at an L level are described, respectively.

First, the input terminal IN at an H level is described. When the inputterminal IN becomes to be at an H level, the transistor 581 is turnedoff. The output terminal OUT is electrically connected to the firstpower supply through the transistor 582, and the potential of the outputterminal OUT drops. The potential of the output terminal OUT at thistime becomes a value which is the sum of the power supply potential VSSand the absolute value of the threshold voltage Vth582 of the transistor582 (VSS+|Vth582|), so that the output terminal OUT becomes to be at anL level.

Next, the input terminal IN at an L level is described. When the inputterminal IN becomes to be at an L level, the transistor 581 is turnedon. The output terminal OUT is electrically connected to the secondpower supply through the transistor 581 and is electrically connected tothe first power supply through the transistor 582, and thus, thepotential of the output terminal OUT rises. The potential of the outputterminal OUT at this time is determined by an operating point of thetransistor 581 and the transistor 582, so that the output terminal OUTbecomes to be at an H level.

The transistor 582 does not necessarily have rectifying properties; anyelement can be used as long as a voltage is generated in the elementwhen a current is supplied thereto. For example, a resistor 621 may beconnected as a substitute for the transistor 582 as in an invertercircuit 620 in FIG. 62.

Here, functions of the transistor 581 and the transistor 582 aredescribed below.

The transistor 581 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with a potential of the input terminal IN. When the inputterminal IN is at an L level, the transistor 581 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 582 has a function as a diode.

FIG. 59 shows another mode of the inverter circuit 211. An invertercircuit 590 shown in FIG. 59 includes a transistor 591, a transistor592, a transistor 593, and a capacitor 594 having two electrodes. Notethat the capacitor 594 is not necessarily provided.

As shown in the inverter circuit 590 in FIG. 59, a first terminal of thetransistor 591 is connected to the second power supply; a secondterminal of the transistor 591 is connected to a second terminal of thetransistor 592, a second electrode of the capacitor 594, and the outputterminal OUT; and a gate terminal of the transistor 591 is connected tothe input terminal IN. A first terminal of the transistor 592 isconnected to the first power supply, and a gate terminal of thetransistor 592 is connected to a second terminal of the transistor 593and a first electrode of the capacitor 594. A first terminal isconnected to the first power supply and a gate terminal of thetransistor 593 are connected to the first power supply.

Note that a first power supply, a second power supply, an input terminalIN, and an output terminal OUT which are the same as those shown in FIG.58 can be used as the first power supply, the second power supply, theinput terminal IN, and the output terminal OUT.

In addition, each of the transistors 591 to 593 is a p-channeltransistor.

Operations of the inverter circuit 590 in FIG. 59 in the case where theinput terminal IN is at an H level and in the case where the inputterminal IN is at an L level are described, respectively.

First, the input terminal IN at an H level is described. When the inputterminal IN becomes to be at an H level, the transistor 591 is turnedoff. A potential of the gate terminal of the transistor 592 becomes avalue which is the sum of the power supply potential VSS and theabsolute value of the threshold voltage Vth593 of the transistor 593(VSS+|Vth593|), so that the transistor 592 is on. In addition, the gateterminal of the transistor 592 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 592, and thus, the potentialof the output terminal OUT drops. The potential of the gate terminal ofthe transistor 592 drops to a value which is less than or equal to avalue obtained by subtracting the absolute value of the thresholdvoltage Vth592 of the transistor 592 from the power supply potential VSS(VSS−|Vth592|) by the capacitive coupling of the capacitor 594, so thatthe transistor 592 is continuously kept on. A so-called bootstrapoperation is performed. Accordingly, the potential of the outputterminal OUT at this time becomes VSS, so that the output terminal OUTbecomes to be at an L level.

Next, the input terminal IN at an L level is described. When the inputterminal IN becomes to be at an L level, the transistor 591 is turnedon. The potential of the gate terminal of the transistor 592 becomes avalue of the sum of the power supply potential VSS and the absolutevalue of the threshold voltage Vth593 of the transistor 593(VSS+|Vth593|), so that the transistor 592 is on. In addition, the gateterminal of the transistor 592 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 591 and is electricallyconnected to the first power supply through the transistor 592, and thepotential of the output terminal OUT rises. The potential of the outputterminal OUT at this time is determined by an operating point of thetransistor 591 and the transistor 592, so that the output terminal OUTbecomes to be at an H level.

In this manner, the L-level potential of the output terminal OUT can belowered to the power supply potential VSS of the first power supply bythe bootstrap operation in the inverter circuit 590 in FIG. 59.

Note that a circuit structure of the inverter circuit 590 in FIG. 59 isnot limited to the circuit structure in FIG. 59 as long as the bootstrapoperation can be performed when the input terminal IN is at an H level.When the input terminal IN is at an L level, a potential may be suppliedto the gate terminal of the transistor 592.

For example, a transistor 631 may be additionally provided as in aninverter circuit 630 in FIG. 63. This is because the potential of theoutput terminal OUT can be made VDD when the output terminal OUT is atan H level. That is, since the transistor 631 is turned on when theinput terminal IN is at an L level, the gate terminal of the transistor592 becomes to be at an H level. Then, the transistor 592 is turned offso that the output terminal OUT is electrically connected only to thesecond power supply through the transistor 591.

It is to be noted that the transistor 631 is a p-channel transistor.

As another example, the first terminal of the transistor 593 may beconnected to the input terminal INb as in an inverter circuit 660 inFIG. 66. This is because the potential of the output terminal OUT can bemade VDD when the out terminal OUT is at an H level. That is, since theinput terminal INb becomes to be at an H level when the input terminalIN is at an L level, the gate terminal of the transistor 592 becomes tobe at an H level. Then, the transistor 592 is turned off so that theoutput terminal OUT is electrically connected only to the second powersupply through the transistor 591.

It is to be noted that an inverted signal of the signal of the inputterminal IN is supplied to the input terminal INb. In addition, an inputterminal INb which is the same as that shown in FIG. 36 can be used asthe input terminal INb.

For example, a signal inputted to the input terminal IN may be suppliedto the input terminal INb through an inverter circuit 1251 as shown inFIG. 125. In addition, the inverter circuits shown in FIGS. 58 to 65 canbe applied as the inverter circuit 1251.

Further, the inverter circuit 360 functions also as a tristate buffercircuit by supplying the control signal to the input terminal INb, whichis shown in FIG. 36. Here, the inverter circuit 660 shown in FIG. 66 cansimilarly function also as a tristate buffer circuit by supplying thecontrol signal to the input terminal INb. That is, when the inputterminal IN becomes to be at an H level and the input terminal INbbecomes to be at an H level, the transistor 591 and the transistor 592are turned off, and thus, the output terminal OUT is not connected toany power supplies; therefore, the inverter circuit 660 can functionalso as the tristate buffer circuit.

An application example of FIG. 59 is further described below.

As another example, the first terminal and the gate terminal of thetransistor 593 may be connected to the input terminal INb, and atransistor 631 may be additionally provided as in an inverter circuit690 in FIG. 69. This is because the potential of the output terminal OUTcan be made VDD when the output terminal OUT is at an H level. That is,when the input terminal INb is at an H level, the gate terminal of thetransistor 592 becomes to be at an H level. Then, the transistor 592 isturned off so that the output terminal OUT is electrically connectedonly to the second power supply through the transistor 591.

Note that any element can be used as the capacitor 594 as long as it hascapacitive properties. For example, a transistor 601, a transistor 641,a transistor 671, and a transistor 701 may be connected as a substitutefor the capacitor 594, respectively, as in an inverter circuit 600 inFIG. 60, in an inverter circuit 640 in FIG. 64, in an inverter circuit670 in FIG. 67, and in an inverter circuit 700 in FIG. 70.

Note that the capacitor 594 is not necessarily provided when acapacitance value between the second terminal and the gate terminal ofthe transistor 592 is sufficiently large. For example, the capacitor 594is not required to be connected as in an inverter circuit 610 in FIG.61, in an inverter circuit 650 in FIG. 65, in an inverter circuit 680 inFIG. 68, and in an inverter circuit 710 in FIG. 71.

Here, functions of the transistors 591 to 593, the transistor 601, thetransistor 631, the transistor 641, and the capacitor 594 are describedbelow.

The transistor 591 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN. When the inputterminal IN is at an L level, the transistor 591 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 592 has a function as a switch which determines whetherto connect the first power supply and the output terminal OUT or not.

The transistor 593 has a function as a diode. In addition, thetransistor 593 has a function of making the gate terminal of thetransistor 592 at a floating state.

The transistor 601 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor592. When the input terminal IN is at an H level, the transistor 601 hasa function of lowering the potential of the gate terminal of thetransistor 592.

The transistor 631 has a function as a switch which determines whetherto connect the second power supply and the gate terminal of thetransistor 592 or not in accordance with the potential of the inputterminal IN. When the input terminal IN is at an L level, the transistor631 has a function of supplying the power supply potential VDD to thegate terminal of the transistor 592.

The transistor 641 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor592. When the input terminal IN is at an L level, the transistor 641 hasa function of drop the potential of the gate terminal of the transistor592 by the drop of the potential of the output terminal OUT.

The capacitor 594 has a function for changing the potential of the gateterminal of the transistor 592 in accordance with the potential of theoutput terminal OUT. When the input terminal IN is at an H level, thecapacitor 594 has a function of lowering the potential of the gateterminal of the transistor 592 by the drop of the potential of theoutput terminal OUT.

In this manner, in the inverter circuits in FIGS. 58 to 71, thepotential of the output terminal OUT can be changed freely by changingthe power supply potential VSS when an L-level signal is output. Thatis, the inverter circuits in FIGS. 58 to 71 can operate not only asinverter circuits, but also as level-shift circuits.

Here, some structure examples which can be applied to the NAND circuit221 are described.

FIG. 42 shows one mode of the NAND circuit 221. A NAND circuit 420 inFIG. 42 includes a transistor 421, a transistor 422, and a transistor423.

As shown in the NAND circuit 420 in FIG. 42, a first terminal of thetransistor 421 is connected to the second power supply; a secondterminal of the transistor 421 is connected to a first terminal of thetransistor 422; and a gate terminal of the transistor 421 is connectedto the input terminal IN1. A second terminal of the transistor 422 isconnected to a first terminal of the transistor 423 and the outputterminal OUT, and a gate terminal of the transistor 422 is connected tothe input terminal IN2. A second terminal is connected to the firstpower supply and a gate terminal of the transistor 423 is connected tothe first power supply.

It is to be noted that the power supply potential VDD is supplied to thefirst power supply and the power supply potential VSS is supplied to thesecond power supply. The potential difference (VDD−VSS) between thepower supply potential VDD of the first power supply and the powersupply potential VSS of the second power supply corresponds to a powersupply voltage of the NAND circuit 420. In addition, the power supplypotential VDD is higher than the power supply potential VSS.

It is to be noted that a digital control signal is supplied to each ofthe input terminal IN1 and the input terminal IN2. In addition, theoutput terminal OUT outputs an output signal.

In addition, each of the transistors 421 to 423 is an n-channeltransistor.

Operations of the NAND circuit 420 in FIG. 42 in the cases where theinput terminal IN1 is at an H level and is at an L level, and in thecases where the input terminal IN2 is at an H level and is at an L levelare described, respectively.

First, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 421 is turnedon. When the input terminal IN2 becomes to be at an H level, thetransistor 422 is turned on.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 421 and the transistor 422and is electrically connected to the first power supply through thetransistor 423, and thus, the potential of the output terminal OUTdrops. The potential of the output terminal OUT at this time isdetermined by an operating point of the transistor 421, the transistor422, and the transistor 423, so that the output terminal OUT becomes tobe at an L level.

Next, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an L level are described. When the inputterminal IN1 becomes to be at an H level, the transistor 421 is turnedon. When the input terminal IN2 becomes to be at an L level, thetransistor 422 is turned off.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 423, and the potential of theoutput terminal OUT rises. The potential of the output terminal OUT atthis time becomes a value obtained by subtracting the threshold voltageVth423 of the transistor 423 from the power supply potential VDD(VDD−Vth423), so that the output terminal OUT becomes to be at an Hlevel.

Next, the input terminal IN1 at an L level and the input terminal IN2 atan H level are described. When the input terminal IN1 becomes to be atan L level, the transistor 421 is turned off. When the input terminalIN2 becomes to be at an H level, the transistor 422 is turned on.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 423, and the potential of theoutput terminal OUT rises. The potential of the output terminal OUT atthis time becomes the value obtained by subtracting the thresholdvoltage Vth423 of the transistor 423 from the power supply potential VDD(VDD−Vth423), so that the output terminal OUT becomes to be at an Hlevel.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 421 is turnedoff. When the input terminal IN2 becomes to be at an L level, thetransistor 422 is turned off.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 423, and the potential of theoutput terminal OUT rises. The potential of the output terminal OUT atthis time becomes the value obtained by subtracting the thresholdvoltage Vth423 of the transistor 423 from the power supply potential VDD(VDD−Vth423), so that the output terminal OUT becomes to be at an Hlevel.

Note that the transistor 423 does not necessarily have rectifyingproperties; any element can be used as long as a voltage is generated inthe element when a current is supplied thereto. For example, a resistor461 may be connected as a substitute for the transistor 423 as in aninverter circuit 460 in FIG. 46.

Here, functions of the transistors 421 to 423 are described below.

The transistor 421 has a function as a switch which determines whetherto connect the second power supply and the first terminal of thetransistor 422 or not in accordance with the potential of the inputterminal IN1.

The transistor 422 has a function as a switch which determines whetherto connect the second terminal of the transistor 421 and the outputterminal OUT or not in accordance with the potential of the inputterminal IN2.

The transistor 423 has a function as a diode.

FIG. 43 shows another mode of the NAND circuit 221. A NAND circuit 430shown in FIG. 43 includes a transistor 431, a transistor 432, atransistor 433, a transistor 434, and a capacitor 435.

As shown in the NAND circuit 430 in FIG. 43, a first terminal of thetransistor 431 is connected to the second power supply; a secondterminal of the transistor 431 is connected to a first terminal of thetransistor 432; and a gate terminal of the transistor 431 is connectedto the input terminal IN1. A second terminal of the transistor 432 isconnected to a second terminal of the transistor 433, a second electrodeof the capacitor 435, and the output terminal OUT; and a gate terminalof the transistor 432 is connected to the input terminal IN2. A firstterminal of the transistor 433 is connected to the first power supply,and a gate terminal of the transistor 433 is connected to a secondterminal of the transistor 434 and a first electrode of the capacitor435. A first terminal of the transistor 434 is connected to the firstpower supply and a gate terminal of the transistor 434 is connected tothe first power supply.

Note that a first power supply, a second power supply, an input terminalIN1, an input terminal IN2, and an output terminal OUT which are thesame as those shown in FIG. 42 can be used as the first power supply,the second power supply, the input terminal IN, and the output terminalOUT.

In addition, each of the transistors 431 to 434 is an n-channeltransistor.

Operations of the NAND circuit 430 in FIG. 43 in the cases where theinput terminal IN1 is at an H level and is at an L level, and in thecases where the input terminal IN2 is at an H level and is at an L levelare described, respectively.

First, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 431 is turnedon. When the input terminal IN2 becomes to be at an H level, thetransistor 432 is turned on. A potential of the gate terminal of thetransistor 433 becomes to be at a value obtained by subtracting thethreshold voltage Vth434 of the transistor 434 from the power supplypotential VDD (VDD−Vth434), so that the transistor 433 is on.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 431 and the transistor 432and is electrically connected to the first power supply through thetransistor 433, and thus, the potential of the output terminal OUTlowers. The potential of the output terminal OUT at this time isdetermined by an operating point of the transistor 431, the transistor432, and the transistor 433, so that the output terminal OUT becomes tobe at an L level.

Next, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 431 is turnedon. When the input terminal IN2 becomes to be at an L level, thetransistor 432 is turned off. The potential of the gate terminal of thetransistor 433 becomes the value obtained by subtracting the thresholdvoltage Vth434 of the transistor 434 from the power supply potential VDD(VDD−Vth434), so that the transistor 433 is on. In addition, the gateterminal of the transistor 433 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 433, and the potential of theoutput terminal OUT rises. The potential of the gate terminal of thetransistor 433 rises to a value which is greater than or equal to thesum of the power supply potential VDD and the threshold voltage Vth433of the transistor 433 by the capacitive coupling of the capacitor 435,so that the transistor 433 is continuously kept on. A so-calledbootstrap operation is performed. Accordingly, the potential of theoutput terminal OUT at this time becomes VDD, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 431 is turnedoff. When the input terminal IN2 becomes to be at an H level, thetransistor 432 is turned on. The potential of the gate terminal of thetransistor 433 becomes the value obtained by subtracting the thresholdvoltage Vth434 of the transistor 434 from the power supply potential VDD(VDD−Vth434), so that the transistor 433 is on. In addition, the gateterminal of the transistor 433 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 433, and the potential of theoutput terminal OUT rises. The potential of the gate terminal of thetransistor 433 rises to a value which is greater than or equal to thesum of the power supply potential VDD and the threshold voltage Vth433of the transistor 433 by the capacitive coupling of the capacitor 435,so that the transistor 433 is continuously kept on. A so-calledbootstrap operation is performed. Accordingly, the potential of theoutput terminal OUT at this time becomes VDD, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 431 is turnedoff. When the input terminal IN2 becomes to be at an L level, thetransistor 432 is turned off. The potential of the gate terminal of thetransistor 433 becomes the value obtained by subtracting the thresholdvoltage Vth434 of the transistor 434 from the power supply potential VDD(VDD−Vth434), so that the transistor 433 is on. In addition, the gateterminal of the transistor 433 is in a floating state.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 433, and the potential of theoutput terminal OUT rises. The potential of the gate terminal of thetransistor 433 rises to a value which is greater than or equal to thesum of the power supply potential VDD and the threshold voltage Vth433of the transistor 433 by the capacitive coupling of the capacitor 435,so that the transistor 433 is continuously kept on. A so-calledbootstrap operation is performed. Accordingly, the potential of theoutput terminal OUT at this time becomes VDD, so that the outputterminal OUT becomes to be at an H level.

In this manner, the H-level potential of the output terminal OUT can beraised to the power supply potential VDD of the first power supply bythe bootstrap operation in the inverter circuit 430 in FIG. 43.

Note that a circuit structure of the NAND circuit 430 in FIG. 43 is notlimited to the circuit structure in FIG. 43 as long as the bootstrapoperation can be performed when the input terminal IN1 or the inputterminal IN2 is at an L level. When the input terminal IN1 and the inputterminal IN2 are an H level, a potential may be supplied to the gateterminal of the transistor 433.

For example, a transistor 471 and a transistor 472 may be additionallyprovided as in a NAND circuit 470 in FIG. 47. This is because thepotential of the output terminal OUT can be made VSS when the outputterminal OUT is at an L level. That is, since the transistor 471 and thetransistor 472 are turned on when the input terminal IN1 and the inputterminal IN2 are at an H level, the gate terminal of the transistor 433becomes to be at an L level. Then, the transistor 433 is turned off sothat the output terminal OUT is electrically connected only to thesecond power supply through the transistor 431 and the transistor 432.

It is to be noted that each of the transistor 471 and the transistor 472is an n-channel transistor.

Note that any element can be used as the capacitor 435 as long as it hascapacitive properties. For example, a transistor 441 and a transistor481 may be connected as a substitute for the capacitor 435,respectively, as in a NAND circuit 440 in FIG. 44 and in a NAND circuit480 in FIG. 48.

Note that the capacitor 435 is not necessarily provided when acapacitance value between the second terminal and the gate terminal ofthe transistor 433 is sufficiently large. For example, the capacitor 435is not required to be connected as in a NAND circuit 450 in FIG. 45 andin a NAND circuit 490 in FIG. 49.

Here, functions of the transistors 431 to 433, the transistor 441, thetransistor 471, the transistor 472, the transistor 481, and thecapacitor 435 are described below.

The transistor 431 has a function as a switch which determines whetherto connect the second power supply and the first terminal of thetransistor 432 or not in accordance with the potential of the inputterminal IN1.

The transistor 432 has a function as a switch which determines whetherto connect the second terminal of the transistor 432 and the outputterminal OUT or not in accordance with the potential of the inputterminal IN2.

The transistor 433 has a function as a switch which determines whetherto connect the first power supply and the output terminal OUT or not.

The transistor 434 has a function as a diode. In addition, thetransistor 434 has a function of making the gate terminal of thetransistor 433 into a floating state.

The transistor 441 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor433. When the input terminal IN1 or the input terminal IN2 is at an Llevel, the transistor 441 has a function of raising the potential of thegate terminal of the transistor 433.

The transistor 471 has a function as a switch which determines whetherto connect the second power supply and a first terminal of thetransistor 472 or not in accordance with the potential of the inputterminal IN1.

The transistor 472 has a function as a switch which determines whetherto connect a first terminal of the transistor 471 and the gate terminalof the transistor 433 or not in accordance with the potential of theinput terminal IN2.

The transistor 481 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor433. When the input terminal IN1 or the input terminal IN2 is at an Llevel, the transistor 481 has a function of raising the potential of thegate terminal of the transistor 433.

The capacitor 435 has a function for changing the potential of the gateterminal of the transistor 433 in accordance with the potential of theoutput terminal OUT. When the input terminal IN1 or the input terminalIN2 is at an L level, the capacitor 435 has a function of raising thepotential of the gate terminal of the transistor 433.

In this manner, in the NAND circuits in FIGS. 42 to 49, the potential ofthe output terminal OUT can be changed freely by changing the powersupply potential VDD when an H-level signal is output. That is, the NANDcircuits in FIGS. 42 to 49 can operate not only as inverter circuits,but also as level-shift circuits.

Although the NAND circuits formed by using all n-channel transistors aredescribed in FIGS. 42 to 49, the NAND circuits may be formed by usingall p-channel transistors as well. Here, NAND circuits formed by usingall p-channel transistors are shown in FIGS. 80 to 87.

FIG. 80 shows another mode of the NAND circuit 221. A NAND circuit 800in FIG. 80 includes a transistor 801, a transistor 802, and a transistor803.

As shown in the NAND circuit 800 in FIG. 80, a first terminal of thetransistor 801 is connected to the second power supply; a secondterminal of the transistor 801 is connected to a second terminal of thetransistor 802, a second terminal of the transistor 803, and the outputterminal OUT; and a gate terminal of the transistor 801 is connected tothe input terminal IN1. A first terminal of the transistor 802 isconnected to the second power supply, and a gate terminal of thetransistor 802 is connected to the input terminal IN2. A first terminalof the transistor 803 is connected to the first power supply and a gateterminal of the transistor 803 is connected to the first power supply.

It is to be noted that the power supply potential VSS is supplied to thefirst power supply and the power supply potential VDD is supplied to thesecond power supply. The potential difference (VDD−VSS) between thepower supply potential VSS of the first power supply and the powersupply potential VDD of the second power supply corresponds to a powersupply voltage of the NAND circuit 800. In addition, the power supplypotential VDD is higher than the power supply potential VSS.

It is to be noted that a digital control signal is supplied to each ofthe input terminal IN1 and the input terminal IN2. In addition, theoutput terminal OUT outputs an output signal.

In addition, each of the transistors 801 to 803 is a p-channeltransistor.

Operations of the NAND circuit 800 in FIG. 80 in the cases where theinput terminal IN1 is at an H level and is at an L level, and in thecases where the input terminal IN2 is at an H level and is at an L levelare described, respectively.

Next, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 801 is turnedoff. When the input terminal IN2 becomes to be at an H level, thetransistor 802 is turned off.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 803, and the potential of theoutput terminal OUT drops. The potential of the output terminal OUT atthis time becomes a value which is the sum of the power supply potentialVSS and the absolute value of the threshold voltage Vth803 of thetransistor 803 (VSS+|Vth803|), so that the output terminal OUT becomesto be at an L level.

Next, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an L level are described. When the inputterminal IN1 becomes to be at an H level, the transistor 801 is turnedoff. When the input terminal IN2 becomes to be at an L level, thetransistor 802 is turned on.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 802 and is electricallyconnected to the first power supply through the transistor 803, andthus, the potential of the output terminal OUT rises. The potential ofthe output terminal OUT at this time is determined by an operating pointof the transistor 802 and the transistor 803, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 801 is turnedon. When the input terminal IN2 becomes to be at an H level, thetransistor 802 is turned off.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 801 and is electricallyconnected to the first power supply through the transistor 803, andthus, the potential of the output terminal OUT rises. The potential ofthe output terminal OUT at this time is determined by an operating pointof the transistor 801 and the transistor 803, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 801 is turnedon. When the input terminal IN2 becomes to be at an L level, thetransistor 802 is turned on.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 801, is electricallyconnected to the second power supply through the transistor 802 and iselectrically connected to the first power supply through the transistor803, and thus, the potential of the output terminal OUT rises. Thepotential of the output terminal OUT at this time is determined by anoperating point of the transistor 801, the transistor 802, and thetransistor 803, so that the output terminal OUT becomes to be at an Hlevel.

Note that the transistor 803 does not necessarily have rectifyingproperties; any element can be used as long as a voltage is generated inthe element when a current is supplied thereto. For example, a resistor841 may be connected as a substitute for the transistor 803 as in a NANDcircuit 840 in FIG. 84.

Here, functions of the transistors 801 to 803 are described below.

The transistor 801 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal M. When the inputterminal IN1 is at an L level, the transistor 801 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 802 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN2. When the inputterminal IN2 is at an L level, the transistor 802 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 803 has a function as a diode.

FIG. 81 shows another mode of the NAND circuit 221. A NAND circuit 810shown in FIG. 81 includes a transistor 811, a transistor 812, atransistor 813, a transistor 814, and a capacitor 815.

As shown in the NAND circuit 810 in FIG. 81, a first terminal of thetransistor 811 is connected to the second power supply; a secondterminal of the transistor 811 is connected to a second terminal of thetransistor 812, a second terminal of the transistor 813, and a firstelectrode of the capacitor 815; and a gate terminal of the transistor811 is connected to the input terminal IN1. A first terminal of thetransistor 812 is connected to the second power supply, and a gateterminal of the transistor 812 is connected to the input terminal IN2. Afirst terminal of the transistor 813 is connected to the first powersupply, and a gate terminal of the transistor 813 is connected to asecond terminal of the transistor 814 and a second electrode of thecapacitor 815. A first terminal of the transistor 814 is connected tothe first power supply and a gate terminal of the transistor 814 isconnected to the first power supply.

Note that a first power supply, a second power supply, an input terminalIN1, an input terminal IN2, and an output terminal OUT which are thesame as those shown in FIG. 80 can be used as the first power supply,the second power supply, the input terminal IN, and the output terminalOUT.

In addition, each of the transistors 811 to 814 is a p-channeltransistor.

Operations of the NAND circuit 810 in FIG. 81 in the cases where theinput terminal IN1 is at an H level and is at an L level, and in thecases where the input terminal IN2 is at an H level and is at an L levelare described, respectively.

First, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 811 is turnedoff. When the input terminal IN2 becomes to be at an H level, thetransistor 812 is turned off. A potential of the gate terminal of thetransistor 813 becomes a value of the sum of the power supply potentialVSS and the absolute value of the threshold voltage Vth814 of thetransistor 814 (VSS+|Vth814|), so that the transistor 813 is on. Inaddition, the gate terminal of the transistor 813 is in a floatingstate.

Accordingly, the output terminal OUT is electrically connected to thefirst power supply through the transistor 813, and the potential of theoutput terminal OUT drops. The potential of the gate terminal of thetransistor 813 drops to a value which is less than or equal to a valueobtained by subtracting the threshold voltage Vth813 of the transistor813 from the power supply potential VSS by the capacitive coupling ofthe capacitor 815, so that the transistor 813 is continuously kept on. Aso-called bootstrap operation is performed. The potential of the outputterminal OUT at this time becomes VSS, so that the output terminal OUTbecomes to be at an L level.

Next, the case where the input terminal IN1 is at an H level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an H level, the transistor 811 is turnedoff. When the input terminal IN2 becomes to be at an L level, thetransistor 812 is turned on. The potential of the gate terminal of thetransistor 813 becomes the value of the sum of the power supplypotential VSS and the absolute value of the threshold voltage Vth814 ofthe transistor 814 (VSS+|Vth814|), so that the transistor 813 is on. Inaddition, the gate terminal of the transistor 813 is in a floatingstate.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 812 and is electricallyconnected to the first power supply through the transistor 813, andthus, the potential of the output terminal OUT rises. The potential ofthe output terminal OUT at this time is determined by an operating pointof the transistor 812 and the transistor 813, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an H level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 811 is turnedon. When the input terminal IN2 becomes to be at an H level, thetransistor 812 is turned off. The potential of the gate terminal of thetransistor 813 becomes the value of the sum of the power supplypotential VSS and the absolute value of the threshold voltage Vth814 ofthe transistor 814 (VSS+|Vth814|), so that the transistor 813 is on. Inaddition, the gate terminal of the transistor 813 is in a floatingstate.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 811 and is electricallyconnected to the first power supply through the transistor 813, andthus, the potential of the output terminal OUT rises. The potential ofthe output terminal OUT at this time is determined by an operating pointof the transistor 811 and the transistor 813, so that the outputterminal OUT becomes to be at an H level.

Next, the case where the input terminal IN1 is at an L level and theinput terminal IN2 is at an L level is described. When the inputterminal IN1 becomes to be at an L level, the transistor 811 is turnedon. When the input terminal IN2 becomes to be at an L level, thetransistor 812 is turned on. The potential of the gate terminal of thetransistor 813 becomes the value of the sum of the power supplypotential VSS and the absolute value of the threshold voltage Vth814 ofthe transistor 814 (VSS+|Vth814|), so that the transistor 813 is on. Inaddition, the gate terminal of the transistor 813 is in a floatingstate.

Accordingly, the output terminal OUT is electrically connected to thesecond power supply through the transistor 811, is electricallyconnected to the second power supply through the transistor 812 and iselectrically connected to the first power supply through the transistor813, and thus, the potential of the output terminal OUT rises. Thepotential of the output terminal OUT at this time is determined by anoperating point of the transistor 811, the transistor 812, and thetransistor 813, so that the output terminal OUT becomes to be at an Hlevel.

In this manner, the L-level potential of the output terminal OUT can belowered to the power supply potential VSS of the first power supply bythe bootstrap operation in the NAND circuit 810 in FIG. 81.

Note that a circuit structure of the NAND circuit 810 in FIG. 81 is notlimited to the circuit structure in FIG. 81 as long as the bootstrapoperation can be performed when the input terminal IN1 and the inputterminal IN2 are at an H level. When the input terminal IN1 or the inputterminal IN2 is at an L level, a potential may be supplied to the gateterminal of the transistor 813.

For example, a transistor 851 and a transistor 852 may be additionallyprovided as in a NAND circuit 850 in FIG. 85. This is because thepotential of the output terminal OUT can be made VDD when the outputterminal OUT is at an H level. That is, since the transistor 851 or thetransistor 852 is turned on when the input terminal IN1 or the inputterminal IN2 is at an L level, the gate terminal of the transistor 813becomes to be at an H level. Then, the transistor 813 is turned off sothat the output terminal OUT is electrically connected only to thesecond power supply through the transistor 811 or the transistor 812.

It is to be noted that each of the transistor 851 and the transistor 852is a p-channel transistor.

Note that any element can be used as the capacitor 815 as long as it hascapacitive properties. For example, a transistor 821 and a transistor861 may be connected as a substitute for the capacitor 815,respectively, as in a NAND circuit 820 in FIG. 82 and in a NAND circuit860 in FIG. 86.

Note that the capacitor 815 is not necessarily provided when acapacitance value between the second terminal and the gate terminal ofthe transistor 813 is sufficiently large. For example, the capacitor 815is not required to be connected as in a NAND circuit 830 in FIG. 83 andin a NAND circuit 870 in FIG. 87.

Here, functions of the transistors 811 to 814, the transistor 821, thetransistor 851, the transistor 852, the transistor 861, and thecapacitor 815 are described below.

The transistor 811 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN1. When the inputterminal IN1 is at an L level, the transistor 811 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 812 has a function as a switch which determines whetherto connect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN2. When the inputterminal IN2 is at an L level, the transistor 812 has a function ofsupplying the power supply potential VDD to the output terminal OUT.

The transistor 813 has a function as a switch which determines whetherto connect the first power supply and the output terminal OUT or not.

The transistor 814 has a function as a diode. In addition, thetransistor 814 has a function of making the gate terminal of thetransistor 813 at a floating state.

The transistor 821 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor813. When the input terminal IN1 and the input terminal IN2 are at an Hlevel, the transistor 821 has a function of lowering the potential ofthe gate terminal of the transistor 813.

The transistor 851 has a function as a switch which determines whetherto connect the second power supply and the gate terminal of thetransistor 813 or not in accordance with the potential of the inputterminal IN1. When the input terminal IN1 is at an L level, thetransistor 851 has a function of supplying the power supply potentialVDD to the gate terminal of the transistor 813.

The transistor 852 has a function as a switch which determines whetherto connect the second power supply and the gate terminal of thetransistor 813 or not in accordance with the potential of the inputterminal IN2. When the input terminal IN2 is at an L level, thetransistor 852 has a function of supplying the power supply potentialVDD to the gate terminal of the transistor 813.

The transistor 861 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor813. When the input terminal IN1 and the input terminal IN2 are at an Hlevel, the transistor 861 has a function of lowering the potential ofthe gate terminal of the transistor 813.

The capacitor 815 has a function for changing the potential of the gateterminal of the transistor 813 in accordance with the potential of theoutput terminal OUT. When the input terminal IN1 or the input terminalIN2 is at an H level, the capacitor 815 has a function of lowering thepotential of the gate terminal of the transistor 813.

In this manner, in the NAND circuits in FIGS. 81 to 87, the potential ofthe output terminal OUT can be changed freely by changing the powersupply potential VSS when an L-level signal is output. That is, the NANDcircuits in FIGS. 81 to 87 can operate not only as NAND circuits, butalso as level-shift circuits.

Here, some structure examples which can be applied to the NOR circuit231 are described.

FIG. 50 shows one mode of the NOR circuit 231. A NOR circuit 500 in FIG.50 includes a transistor 501, a transistor 502, and a transistor 503.

As shown in the NOR circuit 500 of FIG. 50, a first terminal of thetransistor 501 is connected to the second power supply. A secondterminal of the transistor 501 is connected to a second terminal of thetransistor 502, a second terminal of the transistor 503, and the outputterminal OUT. A gate terminal of the transistor 501 is connected to theinput terminal IN1. A first terminal of the transistor 502 is connectedto the second power supply. A gate terminal of the transistor 502 isconnected to the input terminal IN2. A first terminal of the transistor503 is connected to the first power supply. A gate terminal of thetransistor 503 is connected to the first power supply.

Note that the power supply potential VDD is supplied to the first powersupply and the power supply potential VSS is supplied to the secondpower supply. The potential difference (VDD−VSS) between the powersupply potential VDD of the first power supply and the power supplypotential VSS of the second power supply corresponds to a power supplyvoltage of the NOR circuit 500. Further, the power supply potential VDDis higher than the power supply potential VSS.

Note that a digital control signal is supplied to each of the inputterminal IN1 and the input terminal IN2. In addition, the outputterminal OUT outputs an output signal.

Moreover, each of the transistors 501 to 503 is an n-channel transistor.

Operations of the NOR circuit 500 in FIG. 50 in the case where the inputterminal IN1 is at an H level, the case where the input terminal IN1 isat an L level, the case where the input terminal IN2 is at an H level,and the case where the input terminal IN2 is at an L level aredescribed, respectively.

First, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2′ is at an H level. When the inputterminal IN1 becomes an H level, the transistor 501 is turned on. Whenthe input terminal IN2 becomes an H level, the transistor 502 is turnedon.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 501 and the transistor 502, and tothe first power supply through the transistor 503; therefore, thepotential of the output terminal OUT is lowered. The potential of theoutput terminal OUT at this time is determined by an operating point ofthe transistor 501, the transistor 502, and the transistor 503, and theoutput terminal OUT becomes an L level.

Next, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an H level, the transistor 501 is turned on. Whenthe input terminal IN2 becomes an L level, the transistor 502 is turnedoff.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 501 and to the first power supplythrough the transistor 503; therefore, the potential of the outputterminal OUT is lowered. The potential of the output terminal OUT atthis time is determined by an operating point of the transistor 501 andthe transistor 503, and the output terminal OUT becomes an L level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an L level, the transistor 501 is turned off. Whenthe input terminal IN2 becomes an H level, the transistor 502 is turnedon.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 502 and to the first power supplythrough the transistor 503; therefore, the potential of the outputterminal OUT is lowered. The potential of the output terminal OUT atthis time is determined by an operating point of the transistor 502 andthe transistor 503, and the output terminal OUT becomes an L level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an L level, the transistor 501 is turned off. Whenthe input terminal IN2 becomes an L level, the transistor 502 is turnedoff.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 503; therefore, the potential of theoutput terminal OUT rises. The potential of the output terminal OUT atthis time is a value obtained by subtracting the threshold voltageVth503 of the transistor 503 from the power supply potential VDD(VDD−Vth503), and the output terminal OUT becomes an H level.

Note that the transistor 503 is not required to have rectifyingproperties; any element can be used as long as a voltage is generated inthe element when a current is supplied thereto. For example, as shown ina NOR circuit 540 of FIG. 54, a resistor 541 may be connected as asubstitute for the transistor 503.

Here, functions of the transistors 501 to 503 are described below.

The transistor 501 has a function as a switch which selects whether toconnect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN1.

The transistor 502 has a function as a switch which selects whether toconnect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN2.

The transistor 503 has a function as a diode.

FIG. 51 shows another mode of the NOR circuit 231. A NOR circuit 510 inFIG. 51 includes a transistor 511, a transistor 512, a transistor 513, atransistor 514, and a capacitor 515 having two electrodes.

As shown in the NOR circuit 510 of FIG. 51, a first terminal of thetransistor 511 is connected to the second power supply. A secondterminal of the transistor 511 is connected to a second terminal of thetransistor 512, a second terminal of the transistor 513, a secondelectrode of the capacitor 515, and the output terminal OUT. A gateterminal of the transistor 511 is connected to the input terminal IN1. Afirst terminal of the transistor 512 is connected to the second powersupply. A gate terminal of the transistor 512 is connected to the inputterminal IN2. A first terminal of the transistor 513 is connected to thefirst power supply. A gate terminal of the transistor 513 is connectedto a second terminal of the transistor 514 and a first electrode of thecapacitor 515. A first terminal of the transistor 514 is connected tothe first power supply. A gate terminal of the transistor 514 isconnected to the first power supply.

Note that the first power supply, the second power supply, the inputterminal IN1, the input terminal IN2, and the output terminal OUT may besimilar to those in FIG. 50.

Moreover, each of the transistors 511 to 514 is an n-channel transistor.

Operations of the NOR circuit 510 in FIG. 51 in the case where the inputterminal IN1 is at an H level, the case where the input terminal IN1 isat an L level, the case where the input terminal IN2 is at an H level,and the case where the input terminal IN2 is at an L level aredescribed, respectively.

First, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an H level, the transistor 511 is turned on. Whenthe input terminal IN2 becomes an H level, the transistor 512 is turnedon. A potential of the gate terminal of the transistor 513 is a valueobtained by subtracting the threshold voltage Vth514 of the transistor514 from the power supply potential VDD (VDD−Vth514), and the transistor513 is on. Further, the gate terminal of the transistor 513 is in afloating state.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 511 and the transistor 512, and tothe first power supply through the transistor 513; therefore, thepotential of the output terminal OUT is lowered. The potential of theoutput terminal OUT at this time is determined by an operating point ofthe transistor 511, the transistor 512, and the transistor 513, and theoutput terminal OUT becomes an L level.

Next, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an H level, the transistor 511 is turned on. Whenthe input terminal IN2 becomes an L level, the transistor 512 is turnedoff. The potential of the gate terminal of the transistor 513 is a valueobtained by subtracting the threshold voltage Vth514 of the transistor514 from the power supply potential VDD (VDD−Vth514), and the transistor513 is on. Further, the gate terminal of the transistor 513 is in afloating state.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 511 and to the first power supplythrough the transistor 513; therefore, the potential of the outputterminal OUT is lowered. The potential of the output terminal OUT atthis time is determined by the operating point of the transistor 511,the transistor 512, and the transistor 513, and the output terminal OUTbecomes an L level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an L level, the transistor 511 is turned off. Whenthe input terminal IN1 becomes an H level, the transistor 512 is turnedon. The potential of the gate terminal of the transistor 513 is a valueobtained by subtracting the threshold voltage Vth514 of the transistor514 from the power supply potential VDD (VDD−Vth514), and the transistor513 is on. Further, the gate terminal of the transistor 513 is in afloating state.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 512 and to the first power supplythrough the transistor 513; therefore, the potential of the outputterminal OUT is lowered. The potential of the output terminal OUT atthis time is determined by the operating point of the transistor 511,the transistor 512, and the transistor 513, and the output terminal OUTbecomes an L level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an L level, the transistor 511 is turned off. Whenthe input terminal IN2 becomes an L level, the transistor 512 is turnedoff. The potential of the gate terminal of the transistor 513 is a valueobtained by subtracting the threshold voltage Vth514 of the transistor514 from the power supply potential VDD (VDD−Vth514), and the transistor513 is on. Further, the gate terminal of the transistor 513 is in afloating state.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 513; therefore, the potential of theoutput terminal OUT rises. The potential of the gate terminal of thetransistor 513 is increased to a value which is greater than or equal tothe sum of the power supply potential VDD and the threshold voltageVth513 of the transistor 513 in accordance with the capacitive couplingof the capacitor 515, and the transistor 503 continues to be in an onstate. A so-called bootstrap operation is performed. The potential ofthe output terminal OUT at this time is VDD, and the output terminal OUTbecomes an H level.

In this manner, in the NOR circuit 510 of FIG. 51, the potential of theoutput terminal OUT can be increased from an H level to the power supplypotential VDD of the first power supply by the bootstrap operation.

Note that the NOR circuit 510 of FIG. 51 is not limited to a circuitstructure of FIG. 51 as long as the bootstrap operation can be performedwhen the input terminal 1N1 and the input terminal IN2 are at an Llevel. When the input terminal IN1 or the input terminal IN2 is at an Hlevel, a potential may be supplied to the gate terminal of thetransistor 513.

For example, as shown in a NOR circuit 550 of FIG. 55, a transistor 551and a transistor 552 may be added. This is because the potential of theoutput terminal OUT can be VSS when the output terminal OUT is at an Llevel. That is, this is because when one or both of the input terminalIN1 and the input terminal IN2 is/are at an H level, one or both of thetransistor 551 and the transistor 552 is/are turned on; therefore, agate terminal of the transistor 513 becomes an L level, andsubsequently, the transistor 513 is turned off, and the output terminalOUT is electrically connected only to the second power supply throughone or both of the transistor 551 and the transistor 552.

Note that each of the transistors 551 and 552 is an n-channeltransistor.

Note that any element can be used for the capacitor 515 as long as ithas capacitive properties. For example, as shown in a NOR circuit 520 ofFIG. 52 and a NOR circuit 560 of FIG. 56, each of a transistor 521 and atransistor 561 may be connected as a substitute for the capacitor 515.

In addition, the capacitor 515 is not necessarily required if acapacitance value between the second terminal and the gate terminal ofthe transistor 513 is sufficiently large. For example, as shown in a NORcircuit 530 of FIG. 53 and a NOR circuit 570 of FIG. 57, the capacitor515 is not required to be connected.

Here, functions of the transistors 511 to 514, the transistor 521, thetransistor 551, the transistor 552, the transistor 561, and thecapacitor 515 are described below, respectively.

The transistor 511 has a function as a switch which selects whether toconnect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN1. When the inputterminal IN1 is at an H level, the power supply potential VSS issupplied to the output terminal OUT.

The transistor 512 has a function as a switch which selects whether toconnect the second power supply and the output terminal OUT or not inaccordance with the potential of the input terminal IN2. When the inputterminal IN2 is at an H level, the power supply potential VSS issupplied to the output terminal OUT.

The transistor 513 has a function as a switch which selects whether toconnect the first power supply and the output terminal OUT or not.

The transistor 514 has a function as a diode and a function to make thegate terminal of the transistor 513 into a floating state.

The transistor 521 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor513. When the input terminal IN1 and the input terminal IN2 are at an Llevel, the transistor 521 has a function to increase the potential ofthe gate terminal of the transistor 513.

The transistor 551 has a function as a switch which selects whether toconnect the second power supply and the gate terminal of the transistor513 or not in accordance with the potential of the input terminal IN1.When the input terminal IN1 is at an H level, the transistor 551 has afunction to supply the power supply potential VSS to the gate terminalof the transistor 513.

The transistor 552 has a function as a switch which selects whether toconnect the second power supply and the gate terminal of the transistor513 or not in accordance with the potential of the input terminal IN2.When the input terminal IN2 is at an H level, the transistor 552 has afunction to supply the power supply potential VSS to the gate terminalof the transistor 513.

The transistor 561 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor513. When the input terminal IN1 and the input terminal IN2 are at an Llevel, the transistor 561 has a function to increase the potential ofthe gate terminal of the transistor 513.

The capacitor 515 has a function to change the potential of the gateterminal of the transistor 513 in accordance with the potential of theoutput terminal OUT. When the input terminal IN1 and the input terminalIN2 are at an L level, the capacitor 515 has a function to increase thepotential of the gate terminal of the transistor 513.

As described above, in the NOR circuits in FIGS. 50 to 57, the potentialof the output terminal OUT can be freely changed by changing the powersupply potential VDD when an H level signal is output. That is, each ofthe NOR circuits in FIGS. 50 to 57 is not only operated as an invertercircuit but can also be operated as a level-shift circuit.

Although the cases where the NOR circuits in FIGS. 50 to 57 are formedby using all n-channel transistors are described, they may be formed byusing all p-channel transistors. Here, FIGS. 72 to 79 show invertercircuits in the case of forming by using all p-channel transistors.

FIG. 72 shows another mode of the NOR circuit 231. A NOR circuit 720 inFIG. 72 includes a transistor 721, a transistor 722, and a transistor723.

As shown in the NOR circuit 720 of FIG. 72, a first terminal of thetransistor 721 is connected to the second power supply. A secondterminal of the transistor 721 is connected to a first terminal of thetransistor 722. A gate terminal of the transistor 721 is connected tothe input terminal IN1. A second terminal of the transistor 722 isconnected to a second terminal of the transistor 723 and the outputterminal OUT. A gate terminal of the transistor 722 is connected to theinput terminal IN2. A first terminal of the transistor 723 is connectedto the first power supply. A gate terminal of the transistor 723 isconnected to the first power supply.

Note that the power supply potential VSS is supplied to the first powersupply and the power supply potential VDD is supplied to the secondpower supply. The potential difference (VDD−VSS) between the powersupply potential VSS of the first power supply and the power supplypotential VDD of the second power supply corresponds to a power supplyvoltage of the NOR circuit 720. Further, the power supply potential VDDis higher than the power supply potential VSS.

Note that a control signal is supplied to each of the input terminal IN1and the input terminal IN2. In addition, the output terminal OUT outputsan output signal.

Moreover, each of the transistors 721 to 723 is a p-channel transistor.

Operations of the NOR circuit 720 in FIG. 72 in the case where the inputterminal IN1 is at an H level, the case where the input terminal IN1 isL level, the case where the input terminal IN2 is at an H level, and thecase where the input terminal IN2 is at an L level are described,respectively.

First, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an H level, the transistor 721 is turned off. Whenthe input terminal IN2 becomes an H level, the transistor 722 is turnedoff.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 723; therefore, the potential of theoutput terminal OUT is lowered. The potential of the output terminal OUTat this time is a value which is the sum of the power supply potentialVSS and the absolute value of the threshold voltage Vth723 of thetransistor 723 (VSS+|Vth723|), and the output terminal OUT becomes an Llevel.

Next, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an H level, the transistor 721 is turned off. Whenthe input terminal IN2 becomes an L level, the transistor 722 is turnedon.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 723; therefore, the potential of theoutput terminal OUT is lowered. The potential of the output terminal OUTat this time is a value which is the sum of the power supply potentialVSS and the absolute value of the threshold voltage Vth723 of thetransistor 723 (VSS+|Vth723|), and the output terminal OUT becomes an Llevel.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an L level, the transistor 721 is turned on. Whenthe input terminal IN2 becomes an H level, the transistor 722 is turnedoff.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 723; therefore, the potential of theoutput terminal OUT is lowered. The potential of the output terminal OUTat this time is a value which is the sum of the power supply potentialVSS and the absolute value of the threshold voltage Vth723 of thetransistor 723 (VSS+|Vth723|), and the output terminal OUT becomes an Llevel.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an L level, the transistor 721 is turned on. Whenthe input terminal IN2 becomes an L level, the transistor 722 is turnedon.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 721 and the transistor 722, and tothe first power supply through the transistor 723; therefore, thepotential of the output terminal OUT rises. The potential of the outputterminal OUT at this time is determined by an operating point of atransistor 721, a transistor 722, and a transistor 723, and the outputterminal OUT becomes an H level.

Note that the transistor 723 is not required to have rectifyingproperties; any element can be used as long as a voltage is generated inthe element when a current is supplied thereto. For example, as shown ina NOR circuit 760 of FIG. 76, a resistor 761 may be connected as asubstitute for the transistor 723.

Here, functions of the transistors 721 to 723 are described below.

The transistor 721 has a function as a switch which selects whether toconnect the second power supply and the first terminal of the transistor722 or not in accordance with the potential of the input terminal IN1.

The transistor 722 has a function as a switch which selects whether toconnect the second terminal of the transistor 721 and the outputterminal OUT or not in accordance with the potential of the inputterminal IN2.

The transistor 723 has a function as a diode.

FIG. 73 shows another mode of the NOR circuit 231. A NOR circuit 730 inFIG. 73 includes a transistor 731, a transistor 732, a transistor 733, atransistor 734, and a capacitor 735 having two electrodes.

As shown in the NOR circuit 730 of FIG. 73, a first terminal of thetransistor 731 is connected to the second power supply. A secondterminal of the transistor 731 is connected to a first terminal of thetransistor 732. A gate terminal of the transistor 731 is connected tothe input terminal IN1. A second terminal of the transistor 732 isconnected to a second terminal of the transistor 733, a second electrodeof the capacitor 735, and the output terminal OUT. A gate terminal ofthe transistor 732 is connected to the input terminal IN2. A firstterminal of the transistor 733 is connected to the first power supply. Agate terminal of the transistor 733 is connected to a second terminal ofthe transistor 734 and a first electrode of the capacitor 735. A firstterminal of the transistor 734 is connected to the first power supply. Agate terminal of the transistor 734 is connected to the first powersupply.

Note that the first power supply, the second power supply, the inputterminal IN1, the input terminal IN2, and the output terminal OUT may besimilar to those in FIG. 72.

Moreover, each of the transistors 731 to 734 is a p-channel transistor.

Operations of the NOR circuit 730 in FIG. 73 in the case where the inputterminal IN1 is at an H level, the case where the input terminal IN1 isat an L level, the case where the input terminal IN2 is at an H level,and the case where the input terminal IN2 is at an L level aredescribed, respectively.

First, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an H level, the transistor 731 is turned off. Whenthe input terminal IN2 becomes an H level, the transistor 732 is turnedoff. A potential of the gate terminal of the transistor 733 is a valuewhich is the sum of the power supply potential VSS and the absolutevalue of the threshold voltage Vth734 of the transistor 734(VSS+|Vth734|), and the transistor 733 is on. Further, the gate terminalof the transistor 733 is in a floating state.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 733; therefore, the potential of theoutput terminal OUT is lowered. The potential of the gate terminal ofthe transistor 733 is lowered to be less than or equal to a valueobtained by subtracting the absolute value of the threshold voltageVth733 of the transistor 733 from the power supply potential VSS inaccordance with the capacitive coupling of the capacitor 735(VSS−|Vth733|), and the transistor 733 continues to be in an on state. Aso-called bootstrap operation is performed. The potential of the outputterminal OUT at this time is VSS, and the output terminal OUT becomes anL level.

Next, the case is described where the input terminal IN1 is at an Hlevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an H level, the transistor 731 is turned off. Whenthe input terminal IN2 becomes an L level, the transistor 732 is turnedon. The potential of the gate terminal of the transistor 733 is a valuewhich is the sum of the power supply potential VSS and the absolutevalue of the threshold voltage Vth734 of the transistor 734(VSS+|Vth734|), and the transistor 733 is on. Further, the gate terminalof the transistor 733 is in a floating state.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 733; therefore, the potential of theoutput terminal OUT is lowered. The potential of the gate terminal ofthe transistor 733 is lowered to be less than or equal to a valueobtained by subtracting the absolute value of the threshold voltageVth733 of the transistor 733 from the power supply potential VSS inaccordance with the capacitive coupling of the capacitor 735(VSS−Vth733|), and the transistor 733 continues to be in an on state. Aso-called bootstrap operation is performed. The potential of the outputterminal OUT at this time is VSS, and the output terminal OUT becomes anL level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an H level. When the inputterminal IN1 becomes an L level, the transistor 731 is turned on. Whenthe input terminal IN2 becomes an H level, the transistor 732 is turnedoff. The potential of the gate terminal of the transistor 733 is a valuewhich is the sum of the power supply potential VSS and the absolutevalue of the threshold voltage Vth734 of the transistor 734(VSS+|Vth734|), and the transistor 733 is on. Further, the gate terminalof the transistor 733 is in a floating state.

Thus, the output terminal OUT is electrically connected to the firstpower supply through the transistor 733; therefore, the potential of theoutput terminal OUT is lowered. The potential of the gate terminal ofthe transistor 733 is lowered to be less than or equal to a valueobtained by subtracting the absolute value of the threshold voltageVth733 of the transistor 733 from the power supply potential VSS inaccordance with the capacitive coupling of the capacitor 735(VSS−|Vth733|), and the transistor 733 continues to be in an on state. Aso-called bootstrap operation is performed. The potential of the outputterminal OUT at this time is VSS, and the output terminal OUT becomes anL level.

Next, the case is described where the input terminal IN1 is at an Llevel and the input terminal IN2 is at an L level. When the inputterminal IN1 becomes an L level, the transistor 731 is turned on. Whenthe input terminal IN2 becomes an L level, the transistor 732 is turnedon. The potential of the gate terminal of the transistor 733 is a valuewhich is the sum of the power supply potential VSS and the absolutevalue of the threshold voltage Vth734 of the transistor 734(VSS+|Vth734|), and the transistor 733 is on. Further, the gate terminalof the transistor 733 is in a floating state.

Thus, the output terminal OUT is electrically connected to the secondpower supply through the transistor 731 and the transistor 732, and tothe first power supply through the transistor 733; therefore, thepotential of the output terminal OUT is increased. The potential of theoutput terminal OUT at this time is determined by an operating point ofthe transistor 731, the transistor 732, and the transistor 733, and theoutput terminal OUT becomes an H level.

In this manner, in the NOR circuit 730 of FIG. 73, the potential of theoutput terminal OUT can be lowered from an L level to the power supplypotential VSS of the first power supply by the bootstrap operation.

Note that the NOR circuit 730 of FIG. 73 is not limited to a circuitstructure of FIG. 73 as long as the bootstrap operation can be performedwhen the input terminal IN1 or the input terminal IN2 is at an H level.When the input terminal IN1 and the input terminal IN2 are at an Llevel, a potential may be supplied to the gate terminal of thetransistor 733.

For example, as shown in a NOR circuit 770 of FIG. 77, a transistor 771and a transistor 772 may be added. This is because the potential of theoutput terminal OUT can be VDD when the output terminal OUT is at an Hlevel. That is, this is because when the input terminal IN1 and theinput terminal IN2 are at an L level, the transistor 771 and thetransistor 772 are turned on; therefore, the gate terminal of thetransistor 733 becomes an H level, and subsequently, the transistor 733is turned off, and the output terminal OUT is electrically connectedonly to the second power supply through the transistor 731 or thetransistor 732.

Note that each of the transistors 771 and 772 is a p-channel transistor.

Note that any element can be used for the capacitor 735 as long as ithas capacitive properties. For example, as shown in a NOR circuit 740 ofFIG. 74 and a NOR circuit 780 of FIG. 78, each of a transistor 741 and atransistor 781 may be connected as a substitute for the capacitor 735.

In addition, the capacitor 735 is not necessarily required if acapacitance value between the second terminal and the gate terminal ofthe transistor 733 is sufficiently large. For example, as shown in a NORcircuit 750 of FIG. 75 and a NOR circuit 790 of FIG. 79, the capacitor735 is not required to be connected.

Here, functions of the transistors 731 to 734, the transistor 741, thetransistor 771, the transistor 772, the transistor 781, and thecapacitor 735 are described below.

The transistor 731 has a function as a switch which selects whether toconnect the second power supply and the first terminal of the transistor732 or not in accordance with the potential of the input terminal IN1.

The transistor 732 has a function as a switch which selects whether toconnect the second terminal of the transistor 731 and the outputterminal OUT or not in accordance with the potential of the inputterminal IN2.

The transistor 733 has a function as a switch which selects whether toconnect the first power supply and the output terminal OUT or not.

The transistor 734 has a function as a diode and a function to put thegate terminal of the transistor 733 in a floating state.

The transistor 741 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor733. When one or both of the input terminal IN1 and the input terminalIN2 is/are at an H level, the transistor 741 has a function to lower thepotential of the gate terminal of the transistor 733.

The transistor 771 has a function as a switch which selects whether toconnect the second power supply and a first terminal of the transistor772 or not in accordance with the potential of the input terminal IN1.

The transistor 772 has a function as a switch which selects whether toconnect a first terminal of the transistor 771 and the gate terminal ofthe transistor 733 or not in accordance with the potential of the inputterminal IN2.

The transistor 781 has a function as a capacitor which is connectedbetween the output terminal OUT and the gate terminal of the transistor733. When one or both of the input terminal IN1 and the input terminalIN2 is/are at an H level, the transistor 781 has a function to lower thepotential of the gate terminal of the transistor 733.

The capacitor 735 has a function to change the potential of the gateterminal of the transistor 733 in accordance with the potential of theoutput terminal OUT. When one or both of the input terminal IN1 and theinput terminal IN2 is/are at an L level, the capacitor 735 has afunction to lower the potential of the gate terminal of the transistor733.

As described above, in the NOR circuits in FIGS. 73 to 78, the potentialof the output terminal OUT can be freely changed by changing the powersupply potential VSS when an L level signal is output. That is, each ofthe NOR circuits in FIGS. 73 to 78 is not only operated as a NANDcircuit but can also be operated as a level-shift circuit.

In addition, circuit structures in FIGS. 28 to 87 are used as theinverter circuit 211, the NAND circuit 221, and the NOR circuit 231;therefore, a margin for operating the shift register circuit 200 isincreased. This is because in the inverter circuit 211, the NAND circuit221, and the NOR circuit 231, a gate terminal of one transistor isconnected to the output terminal SRout. Thus, load capacitance of theoutput terminal SRout is decreased; therefore, a margin for operatingthe shift register circuit 200 can be increased.

In addition, the inverter circuits, the NAND circuits, and the NORcircuits shown in FIGS. 28 to 87 are formed by using transistors havingthe same polarity, respectively. Therefore, when the polarity of thesetransistors is the same as a polarity of other transistors over the samesubstrate, simplification of a manufacturing process can be realized.Accordingly, reduction in manufacturing cost and improvement in yieldcan be realized.

Note that although the power supply potential VDD or the power supplypotential VSS is supplied to the first power supply and the second powersupply shown in FIGS. 28 to 87, the invention is not limited thereto.

For example, a different potential may be supplied to each of the firstpower supply and the second power supply in FIGS. 28 to 87.

As another example, the control signal may be supplied to each of thefirst power supply and the second power supply in FIGS. 28 to 87.

Note that although the control signal is supplied to each of the inputterminals in FIGS. 28 to 87, the invention is not limited thereto.

For example, the power supply voltage may be supplied to the inputterminal in FIGS. 28 to 87.

Note that this embodiment mode can be freely implemented in combinationwith any description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor in theshift register circuit of the invention is turned on at regularintervals, so that a power supply potential to the output terminal issupplied. Therefore, the power supply potential is supplied to theoutput terminal of the shift register circuit through the transistor.Since the transistor is not always on in the non-selection period, thethreshold voltage shift of the transistor can be suppressed. Further,the power supply potential is supplied to the output terminal of theshift register circuit through the transistor at regular intervals.Therefore, the shift register circuit can suppress noise which isgenerated in the output terminal.

Embodiment Mode 4

In this embodiment mode, a structure which is different from the drivercircuit described in Embodiment Mode 3 is described.

As a driver circuit, a structure example which can be applied to asource driver is described with reference to FIGS. 88 to 91. Drivercircuits in FIGS. 88 to 91 can be applied not only to a source driverbut also to any kind of circuit structures.

FIG. 88 shows one mode of a source driver of the invention. The sourcedriver of the invention includes a shift register circuit 880, aplurality of switches SW, and a video signal line 881.

As shown in the source driver of FIG. 88, the video signal line 881 isconnected to a first terminal of the switch SW and a second terminal ofthe switch SW is connected to an output terminal SDout. A controlterminal of the switch SW is connected to an output terminal SRout ofthe shift register circuit 880.

Note that the shift register circuit 880 is similar to that described inEmbodiment Mode 2. Further, the gate driver described in Embodiment Mode3 may be applied to the shift register circuit 880.

Output terminals SRout1 to SRout4 and an output terminal SRoutn of theshift register circuit 880 may be similar to those described inEmbodiment Mode 2.

An output terminal SDout of a first stage of the gate driver of theinvention is denoted by an output terminal SDout1. An output terminalSDout of a second stage is denoted by an output terminal SDout2. Anoutput terminal SDout of a third stage is denoted by an output terminalSDout3. An output terminal SDout of an n-th stage is denoted by anoutput terminal SDoutn.

In the source driver of FIG. 88, a power supply line and a controlsignal line are not shown in the figure for convenience.

In the case where the shift register circuit 880 is formed by using ann-channel transistor, an output signal of the shift register circuit 880is similar to that in the timing chart of FIG. 18. In the case where theshift register circuit 880 is formed by using a p-channel transistor, anoutput signal of the shift register circuit 880 is similar to that inthe timing chart of FIG. 19.

A video signal is supplied to the video signal line 881. The videosignal may be a current or a voltage; and an analog signal or a digitalsignal. The video signal is preferably an analog voltage since a numberof external circuits are for a liquid crystal display device. That is,when the video signal is an analog voltage, an inexpensive conventionalcircuit can be used as the external circuit.

Operations of the source driver in FIG. 88 in the cases where the outputterminal SRout of the shift register circuit 880 is at an H level and anL level are described, respectively.

Note that for convenience, the switch SW in FIG. 88 is turned on whenthe control terminal is at an H level and turned off when the controlterminal is at an L level. Needless to say, the switch SW may be turnedoff when the control terminal is at an H level and turned on when thecontrol terminal is at an L level.

First, the case where the output terminal SRout is at an H level isdescribed. When the output terminal SRout of the shift register circuitbecomes an H level, the switch SW is turned on. When the switch SW isturned on, the video signal line 881 is connected to the output terminalSRout of the source driver through the switch SW.

Therefore, the output terminal SDout of the source driver has the samepotential or the same current as the video signal line 881, and thesource driver outputs a video signal.

Next, the case where the output terminal SRout is at an L level isdescribed. When the output terminal SRout of the shift register circuitbecomes an L level, the switch SW is turned off. When the switch SW isturned off, the video signal line 881 is disconnected from the outputterminal SRout of the source driver.

Therefore, the output terminal SDout of the source driver is notaffected by a potential of the video signal line 881, and the sourcedriver stops outputting the video signal.

As described in Embodiment Mode 2, in the case where the shift registercircuit 880 includes an n-channel transistor, the shift register circuit880 becomes an H level sequentially from the output terminal SRout1.That is, the switch shown in FIG. 88 is turned on sequentially from aswitch SW1 (in a first column) and the output terminals SDout of thesource driver have the same potential or the same current as the videosignal sequentially from the output terminal SDout1 (in a first column).

Note that the source driver shown in FIG. 88 can output different videosignals sequentially from the output terminal SDout1 by changing thevideo signal each time the shift register circuit 880 outputs an H levelsignal.

Note that although each output terminal SRout of the shift registercircuit 880 controls one switch, the invention is not necessarilylimited to this. Each output terminal SRout of the shift registercircuit 880 may control a plurality of switches SW. In this case, aplurality of video signal lines may connect to the first terminals ofthe switches SW, respectively.

For example, as shown in a source driver of FIG. 89, one output terminalSRout of the shift register circuit 880 may control three switches SW.This is because a video signal line 891, a video signal line 892, and avideo signal line 893 are connected to first terminals of the threeswitches, so that three output terminals SDout of the source driver canoutput video signals simultaneously. Therefore, an operating frequencyof the shift register circuit 880 can be low, and thereby powerconsumption of the shift register circuit 880 is reduced.

Note that as the switch SW, an electrical switch or a mechanical switchcan be used, for example. That is, any element which can control a flowof current can be employed and the switch is not limited to a specificelement. A transistor, a diode, or a logic circuit that is a combinationthereof may be employed. When a transistor is used as a switch, apolarity (conductivity type) thereof is not specifically limited sincethe transistor is operated as a mere switch. However, in the case wherean off-current is preferably small, a transistor with a polarity of asmall off-current is preferably used. As a transistor with a smalloff-current, a transistor provided with an LDD region, a transistorhaving a multi-gate structure, or the like may be used. In addition, ann-channel transistor is preferably used when operating in a state wherea potential of a source terminal of the transistor, which operates as aswitch, is close to a low potential side power supply (Vss, GND, 0V, orthe like), whereas a p-channel transistor is preferably used whenoperating in a state where a potential of a source terminal of thetransistor is close to a high potential side power supply (Vdd or thelike). This is because the transistor can easily function as a switchsince the absolute value of a gate-source voltage thereof can be made tobe large. Note that a CMOS type switch may also be applied by using bothan n-channel transistor and a p-channel transistor.

For example, as shown in a source driver of FIG. 90, a transistor 901may be connected as the switch SW. The transistor 901 is controlled tobe turned on and off by the shift register circuit 880. When thetransistor 901 is turned on, an output terminal SDout of the sourcedriver outputs a video signal.

Note that the transistor 901 is an n-channel transistor.

Note that the transistor 901 has a function as a switch which selectswhether to connect the video signal line 881 and the output terminalSDout of the source driver or not in accordance with a potential of theoutput terminal SRout of the shift register circuit 880. When the outputterminal SRout of the shift register circuit 880 is at an H level, thevideo signal is supplied to the output terminal SDout of the sourcedriver by the transistor 901.

Note that the shift register circuit 880 at this time is preferablyformed by using an n-channel transistor. When the shift register circuit880 is formed by using an n-channel transistor, simplification of amanufacturing process can be realized. Therefore, reduction inmanufacturing cost and improvement in yield can be realized.

As another example, as shown in a source driver of FIG. 91, a transistor911 may be connected as the switch SW. The transistor 911 is controlledto be turned on and off by the shift register circuit 880. When thetransistor 911 is turned on, the output terminal SDout of the sourcedriver outputs the video signal.

Note that the transistor 911 is a p-channel transistor.

Note that the transistor 911 has a function as a switch which selectswhether to connect the video signal line 881 and the output terminalSDout of the source driver or not in accordance with the potential ofthe output terminal SRout of the shift register circuit 880. When theoutput terminal SRout of the shift register circuit 880 is at an Llevel, the video signal is supplied to the output terminal SDout of thesource driver by the transistor 911.

Note that the shift register circuit 880 at this time is preferablyformed by using a p-channel transistor. When the shift register circuit880 is formed by using a p-channel transistor, simplification of amanufacturing process can be realized. Therefore, reduction inmanufacturing cost and improvement in yield can be realized.

Note that this embodiment mode can be freely implemented in combinationwith any description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor in theshift register circuit of the invention is turned on at regularintervals, so that a power supply potential to the output terminal issupplied. Therefore, the power supply potential is supplied to theoutput terminal of the shift register circuit through the transistor.Since the transistor is not always on in the non-selection period, thethreshold voltage shift of the transistor can be suppressed. Further,the power supply potential is supplied to the output terminal of theshift register circuit through the transistor at regular intervals.Therefore, the shift register circuit can suppress noise which isgenerated in the output terminal.

Embodiment Mode 5

In this embodiment mode, a layout diagram of the flip-flop circuit shownin Embodiment Mode 1 is described.

FIG. 122 is a layout diagram of the flip-flop circuit 10 shown in FIG.1.

Note that the layout diagram of the flip-flop circuit 10 shown in FIG.122 shows the case where the flip-flop circuit is formed by using atransistor made from amorphous silicon.

The flip-flop circuit in FIG. 122 includes a power supply line 12201, acontrol line 12202, a control line 12203, a control line 12204, acontrol line 12205, a power supply line 12206, an output terminal 12207,the transistor 11, the transistor 12, the transistor 13, the transistor14, the transistor 15, the transistor 16, the transistor 17, and thetransistor 18.

Reference numeral 12208 denotes a semiconductor layer. Reference numeral12209 denotes a gate electrode and a gate wiring layer. Referencenumeral 12210 denotes a second wiring layer. Reference numeral 12211denotes a contact layer.

Connection relations of the flip-flop circuit shown in FIG. 122 isdescribed. As shown in the flip-flop circuit 10, the gate terminal ofthe transistor 11 is connected to the input terminal IN1. The firstterminal of the transistor 11 is connected to the first power supply.The second terminal of the transistor 11 is connected to the gateterminal of the transistor 12, the second terminal of the transistor 14,the gate terminal of the transistor 15, the second terminal of thetransistor 17, and the second electrode of the capacitor 19. The firstterminal of the transistor 15 is connected to the second power supply.The second terminal of the transistor 15 is connected to the secondterminal of the transistor 16 and the gate terminal of the transistor18. The gate terminal and the first terminal of the transistor 16 areconnected to the first power supply. The first terminal of thetransistor 18 is connected to the input terminal IN3. The secondterminal of the transistor 18 is connected to the gate terminal of thetransistor 13 and the gate terminal of the transistor 14. The firstterminal of the transistor 13 is connected to the second power supply.The second terminal of the transistor 13 is connected to the firstelectrode of the capacitor 19, the second terminal of the transistor 12,and the output terminal OUT. The first terminal of the transistor 12 isconnected to the input terminal IN2. The first terminal of thetransistor 14 is connected to the second power supply. The gate terminalof the transistor 17 is connected to the input terminal IN4, and thefirst terminal of the transistor 17 is connected to the second powersupply.

Note that the transistors 11 to 18 in FIG. 122 correspond to thetransistors 11 to 18 in FIG. 1, respectively. The control line 12204,the control line 12202, the control line 12203, and the control line12205 correspond to the input terminals 1N1 to 1N4 in FIG. 1,respectively. The output terminal 12207 corresponds to the outputterminal Out in FIG. 1.

Note that in the layout diagram of the flip-flop circuit 10 in FIG. 122,a channel region of the transistor 15 is U-shaped. Note that asdescribed above, the size of the transistor 15 is required to be large.Therefore, by making the channel region U-shaped like the transistor 15in FIG. 122, the transistor 15 occupying a small area and having a largesize (or a large W/L ratio) can be realized.

Note that line widths of the control line 12202 and the control line12203 are larger than that of the power supply line 12201. In theflip-flop circuit of FIG. 122, a current or a voltage is supplied to theflip-flop circuit more from the control line 12202 and the control line12203 than from the power supply line 12201. Therefore, an effect of avoltage drop of the control line 12202 and the control line 12203 can bereduced when the control line 12202 and the control line 12203 are wide.

Note that although the flip-flop circuit in FIG. 122 is formed using atransistor made from amorphous silicon, the invention is not limited tothis.

For example, as shown in a flip-flop circuit of FIG. 123, the flip-flopcircuit may be formed by using a transistor made from polysilicon.

Here, the case where the flip-flop circuit is formed by using atransistor made from polysilicon is described.

The flip-flop circuit in FIG. 123 includes the power supply line 12201,the control line 12202, the control line 12203, the control line 12204,the control line 12205, the power supply line 12206, the output terminal12207, the transistor 11, the transistor 12, the transistor 13, thetransistor 14, the transistor 15, the transistor 16, the transistor 17,and the transistor 18.

The reference numeral 12208 denotes the semiconductor layer. Thereference numeral 12209 denotes the gate electrode and the gate wiringlayer. The reference numeral 12210 denotes the second wiring layer. Thereference numeral 12211 denotes the contact layer.

Connection relations of the flip-flop circuit shown in FIG. 123 isdescribed. As shown in the flip-flop circuit 10, the gate terminal ofthe transistor 11 is connected to the input terminal IN1. The firstterminal of the transistor 11 is connected to the first power supply.The second terminal of the transistor 11 is connected to the gateterminal of the transistor 12, the second terminal of the transistor 14,the gate terminal of the transistor 15, the second terminal of thetransistor 17, and the second electrode of the capacitor 19. The firstterminal of the transistor 15 is connected to the second power supply.The second terminal of the transistor 15 is connected to the secondterminal of the transistor 16 and the gate terminal of the transistor18. The gate terminal and the first terminal of the transistor 16 areconnected to the first power supply. The first terminal of thetransistor 18 is connected to the input terminal IN3. The secondterminal of the transistor 18 is connected to the gate terminal of thetransistor 13 and the gate terminal of the transistor 14. The firstterminal of the transistor 13 is connected to the second power supply.The second terminal of the transistor 13 is connected to the firstelectrode of the capacitor 19, the second terminal of the transistor 12,and the output terminal OUT. The first terminal of the transistor 12 isconnected to the input terminal IN2. The first terminal of thetransistor 14 is connected to the second power supply. The gate terminalof the transistor 17 is connected to the input terminal IN4, and thefirst terminal of the transistor 17 is connected to the second powersupply.

Note that the power supply line 12201, the control line 12202, thecontrol line 12203, the control line 12204, the control line 12205, thepower supply line 12206, the output terminal 12207, the transistor 11,the transistor 12, the transistor 13, the transistor 14, the transistor15, the transistor 16, the transistor 17, and the transistor 18 may besimilar to those in FIG. 122.

Note that the semiconductor layer 12208, the gate wiring layer 12209 (agate electrode layer), the second wiring layer 12210, and the contactlayer 12211 may be similar to those in FIG. 122.

Note that in the layout diagram of the flip-flop circuit in FIG. 123,the gate terminal of the transistor 13 and the gate terminal of thetransistor 14 are connected to each other thorough the second wiringlayer 12210, and thereby the gate wiring layer 12209 can be shortened.In a manufacturing process of a semiconductor device, it is known thatelectrostatic discharge damage is likely to occur through the gatewiring layer 12209 if the gate wiring layer 12209 is long. Therefore,the gate terminal of the transistor 13 and the gate terminal of thetransistor 14 are connected to each other thorough the second wiringlayer 12210, so that electrostatic discharge damage through the gatewiring layer 12209 can be reduced. Reducing electrostatic dischargedamage offers advantages such as improvement in yield, improvement inproductivity, and long lifetime of a semiconductor device.

Note that the transistor 15 is provided with a plurality of channelregions. By dividing the channel region into a plurality of regions,heat generation of the transistor 15 can be reduced and characteristicsdeterioration of the transistor 15 can be suppressed.

Note that this embodiment mode can be freely implemented in combinationwith any description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor in theshift register circuit of the invention is turned on at regularintervals, so that a power supply potential to the output terminal issupplied. Therefore, the power supply potential is supplied to theoutput terminal of the shift register circuit through the transistor.Since the transistor is not always on in the non-selection period, thethreshold voltage shift of the transistor can be suppressed. Further,the power supply potential is supplied to the output terminal of theshift register circuit through the transistor at regular intervals.Therefore, the shift register circuit can suppress noise which isgenerated in the output terminal.

Embodiment 1

In this embodiment, structures of a display device, a gate driver, asource driver, and the like are described. Note that the semiconductordevice of the invention can be applied to a part of the gate driver orthe source driver.

FIG. 92 shows one mode of a display device to which the invention isapplied. A display device 920 to which the invention is applied includesa pixel region 921, a gate driver 922, a control signal line 923, and anFPC 926. The pixel region 921 includes a pixel. The pixel includes adisplay element and a circuit for controlling the display element.

In FIG. 92, the FPC 926 is connected to the control signal line 923 anda source signal line 924. The gate driver 922 is connected to thecontrol signal line 923 and a gate signal line 925.

Note that as the gate driver 922 similar to those described inEmbodiment Mode 3 can be used.

Further, the number of the gate drivers 922 may be more than one.

As described above, a display device, which is a device including adisplay element, or a light-emitting device, which is a device includinga light-emitting element, can employ various modes or include variouselement. For example, a display medium in which contrast is changed byan electrical or magnetic effect, such as an EL element (an organic ELelement, an inorganic EL element, or an EL element including an organiccompound and an inorganic compound), an electron-emissive element, aliquid crystal element, or electronic ink can be applied. Note thatdisplay devices using an EL element include an EL display; displaydevices using an electron-emissive element include a field emissiondisplay (FED), an SED type flat panel display (Surface-conductionElectron-emitter Display), and the like; display devices using a liquidcrystal element include a liquid crystal display; and display devicesusing electronic ink include electronic paper.

An operation of the display device 920 is briefly described.

The gate driver 922 outputs selection signals sequentially to the pixelregion 921 through the gate signal line 925. An external circuit outputsvideo signals sequentially to the pixel region 921 through the FPC 926and the source signal line 924. The external circuit is not shown in thefigure. In the pixel region 921, an image is displayed by controlling astate of light in accordance with the video signal.

Note that a control signal is supplied to the control signal line 923from the external circuit and the gate driver 922 is controlled by thecontrol signal. For example, a start pulse, a clock signal, an invertedclock signal, or the like is used as the control signal.

Note that the video signal may be a voltage value input or a currentvalue input. For example, when a liquid crystal element is used as thedisplay element, the video signal is preferably a voltage value input.This is because a tilt of the liquid crystal element is controlled by enelectric field, so that the liquid crystal element can be controlledmore easily by a video signal having a voltage value.

Note that the video signal may be either a digital value or an analogvalue. For example, when a liquid crystal element is used as the displayelement, the video signal is preferably an analog value. This is becausea response speed of the liquid crystal element is slow, so that theliquid crystal element can be controlled by supplying the video signalhaving an analog value only once in one frame period.

Note that although the FPC 926 is formed of one FPC 926, the inventionis not necessarily limited to this. The FPC 926 may be divided into aplurality of FPCs.

For example, as shown in the display device 920 of FIG. 93, the FPC 926may be divided into three. This is because even in the case where thedisplay device is large or the case where the number of connectionsbetween the FPC 926 and the display device 920 is large, a conventionalFPC and a conventional FPC pressure bonding device can be used, andthereby manufacturing cost can be reduced. Further, if the connectionbetween the FPC 926 and the display device 920 fails, only an FPC 926which fails to connect needs to be changed; therefore, manufacturingcost can be reduced.

Note that the video signal may be output to the pixel region 921 throughany circuit and any element.

For example, as shown in FIG. 94, the video signal may be output to thepixel region 921 through a signal line control circuit 941. This isbecause when the signal line control circuit 941 has various functions,a structure of the external circuit can be simplified; therefore, costof the display device as a whole can be reduced. Further, the number ofconnections between the FPC 926 and the display device 920 can begreatly reduced.

Note that the video signal and the control signal are supplied to thesignal line control circuit 941 through a control signal line 942.

As described above, various structures can be applied to the displaydevice of the invention.

Note that in this embodiment, although the structures of various displaydevices are shown, a structure of the display device of the invention isnot limited to these display devices.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the gate driver and the sourcedriver provided with the shift register circuit of the invention supplya power supply potential to the output terminal. Therefore, the powersupply potential is supplied to the output terminal of the shiftregister circuit through the transistor. Since the transistor is notalways on in the non-selection period, the threshold voltage shift ofthe transistor can be suppressed. Further, the power supply potential issupplied to the output terminal of the shift register circuit throughthe transistor at regular intervals. Therefore, the shift registercircuit can suppress noise which is generated in the output terminal.

Embodiment 2

Next, a specific structure of the signal line control circuit 941described in Embodiment 1 is described.

As the signal line control circuit 941, the source driver described inEmbodiment Mode 4 can be applied.

FIG. 95 shows one mode of the signal line control circuit 941 differentfrom the source driver described in Embodiment Mode 4. A signal linecontrol circuit 950 in FIG. 95 includes a plurality of switches SW.

As shown in FIG. 95, a video signal line 954 is connected to a firstterminal of a switch SW1, a first terminal of a switch SW2, and a firstterminal of a switch SW3. A second terminal of the switch SW1 isconnected to a source signal line 955. A second terminal of the switchSW2 is connected to a source signal line 956. A second terminal of theswitch SW3 is connected to a source signal line 957. A control terminalof the switch SW1 is connected to a control signal line 951. A controlterminal of the switch SW2 is connected to a control signal line 952. Acontrol terminal of the switch SW3 is connected to a control signal line953. The video signal line 954, the control signal line 951, the controlsignal line 952, and the control signal line 953 are connected to anexternal circuit through an FPC.

Note that a control signal A is supplied to the control signal line 951.A control signal B is supplied to the control signal line 952. A controlsignal C is supplied to the control signal line 953. A video signal issupplied to the video signal line 954.

As described above, an electrical switch or a mechanical switch can beused as the switches SW1 to SW3, for example. That is, any element whichcan control a flow of current can be employed and the switch is notlimited to a specific element. A transistor, a diode, or a logic circuitthat is a combination thereof may be employed. When a transistor is usedas a switch, a polarity (conductivity type) thereof is not specificallylimited since the transistor is operated as a mere switch. However, inthe case where an off-current is preferably small, a transistor with apolarity of a smaller off-current is preferably used. As a transistorwith a small off-current, a transistor provided with an LDD region, atransistor having a multi-gate structure, or the like may be used. Inaddition, an n-channel transistor is preferably used when operating in astate where a potential of a source terminal of the transistor, whichoperates as a switch, is close to a low potential side power supply(Vss, GND, 0V, or the like), whereas a p-channel transistor ispreferably used when operating in a state where a potential of a sourceterminal of the transistor is close to a high potential side powersupply (Vdd or the like). This is because the transistor can easilyfunction as a switch since the absolute value of a gate-source voltagethereof can be made to be large. Note that a CMOS type switch may alsobe applied by using both an n-channel transistor and a p-channeltransistor.

An operation of the signal line control circuit 950 in FIG. 95 isdescribed.

The control signal A, the control signal B, and the control signal C aresignals for turning on the switch SW1, the switch SW2, and the switchSW3 sequentially. A value of the video signal is changed in accordancewith on and off states of the switch SW1, the switch SW2, and the switchSW3.

First, the switch SW1 is turned on by the control signal A. At thistime, the switch SW2 is turned off by the control signal B and theswitch SW3 is turned off by the control signal C. Therefore, the videosignal is supplied to the source signal line 955 through the videosignal line 954 and the switch SW1. Since the switch SW2 and the switchSW3 are off at this time, the video signal is not supplied to the sourcesignal line 956 and the source signal line 957.

Next, the switch SW2 is turned on by the control signal B. At this time,the switch SW1 is turned off by the control signal A and the switch SW3is turned off by the control signal C. Therefore, the video signal issupplied to the source signal line 956 through the video signal line 954and the switch SW2. Since the switch SW1 and the switch SW3 are off atthis time, the video signal is not supplied to the source signal line955 and the source signal line 957.

Next, the switch SW3 is turned on by the control signal C. At this time,the switch SW1 is turned off by the control signal A and the switch SW2is turned off by the control signal B. Therefore, the video signal issupplied to the source signal line 957 through the video signal line 954and the switch SW3. Since the switch SW1 and the switch SW2 are off atthis time, the video signal is not supplied to the source signal line955 and the source signal line 956.

By such an operation as described above, the video signal is supplied tothree lines of the source signal line 955, the source signal line 956,and the source signal line 957 using one video signal line 954. That is,the number of the video signal lines 954 is one third of the number ofthe source signal lines; therefore, the number of connections betweenthe FPC and a display device is greatly reduced. Accordingly, a failureratio of a connection between the FPC and the display device is greatlyreduced.

Note that although the signal line control circuit 950 in FIG. 95includes three switches SW, the invention is not limited to this. Thenumber of the switches is not limited. The number of the control signalsare required to be changed in accordance with the number of switches SW.For example, in the case of providing four switches SW, four controlsignals are provided.

Note that the signal line control circuit 950 in FIG. 95 may be providedwith a period when none of the switches SW1 to SW3 is turned on sinceimage defect such as crosstalk can be reduced. That is, when a new videosignal is supplied to the source signal line, a potential of the sourcesignal line is not changed immediately. This is because when an effectof a previous potential remains in the source signal line in some cases,image defect such as crosstalk occurs. This period is a preparationperiod for writing to a next row.

Note that the control signal A, the control signal B, and the controlsignal C may be supplied by the shift register circuit in EmbodimentMode 2. At this time, the shift register circuit includes three or moreflip-flop circuits. Preferably, the shift register circuit includesthree or more flip-flop circuits and five or less flip-flop circuits.

Note that in the display device 920, the signal line control circuit 950is formed over the same substrate, so that the number of connectionsbetween the FPC and the display device 920 can be further reduced.

As described above, various signal control circuits can be used for thedisplay device of the invention.

Note that in this embodiment, although various signal control circuitsare shown, a signal control circuit to which can be applied to thedisplay device of the invention is not limited to these signal controlcircuits.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the signal control circuitprovided with the shift register circuit of the invention supplies apower supply potential to the output terminal. Therefore, the powersupply potential is supplied to the output terminal of the shiftregister circuit through the transistor. Since the transistor is notalways on in the non-selection period, the threshold voltage shift ofthe transistor can be suppressed. Further, the power supply potential issupplied to the output terminal of the shift register circuit throughthe transistor at regular intervals. Therefore, the shift registercircuit can suppress noise which is generated in the output terminal.

Embodiment 3

Next, a specific structure of the pixel described in Embodiment 1 isdescribed.

FIG. 96 shows one mode of a pixel. A pixel 960 in FIG. 96 includes atransistor 961, a liquid crystal element 962 having two electrodes, anda capacitor 963 having two electrodes.

As shown in the pixel 960 of FIG. 96, a first terminal of the transistor961 is connected to the source signal line 924. A second terminal of thetransistor 961 is connected to a first electrode of the liquid crystalelement 962 and a first electrode of the capacitor 963. A gate terminalof the transistor 961 is connected to the gate signal line 925. A secondelectrode of the liquid crystal element 962 is an opposite electrode964. A second electrode of the capacitor 963 is connected to a commonline 965.

Note that a video signal is supplied to the source signal line 924. Aselection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that a common potential is supplied to the common line 965. Asubstrate potential is supplied to the opposite electrode 964. Thecommon potential and the substrate potential are constant potentials.

The transistor 961 is an n-channel transistor.

Operations of the pixel 960 in FIG. 96 in the case where the selectionsignal is supplied to the gate signal line 925 (H level) and the casewhere the selection signal is not supplied (L level) are described,respectively. A first period is a period when the selection signal issupplied to the gate signal line 925. A second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anH level, and the transistor 961 is turned on. The source signal line 924is electrically connected to the first electrode of the liquid crystalelement 962 and the first electrode of the capacitor 963. Potentials ofthe first electrode of the liquid crystal element 962 and the firstelectrode of the capacitor 963 become the same potential as that of thesource signal line 924.

Here, the potential of the source signal line 924 corresponds to thevideo signal.

Light transmittance of the liquid crystal element 962 is determined by apotential corresponding to the video signal. The potential correspondingto the video signal is held in the capacitor 963.

Next, the second period is described. The gate signal line 925 is at anL level, and the transistor 961 is turned off. The source signal line924 is electrically disconnected from the first electrode of the liquidcrystal element 962 and the first electrode of the capacitor 963.Therefore, the potential corresponding to the video signal inputpreviously is maintained as the potentials of the first electrode of theliquid crystal element 962 and the first electrode of the capacitor 963,and thereby the light transmittance of the liquid crystal element 962 ismaintained as well.

Here, functions of the transistor 961 and the capacitor 963 aredescribed below.

The transistor 961 has a function as a switch which selects whether thesource signal line 924 is connected to the first electrode of the liquidcrystal element 962 and the first electrode of the capacitor 963 inaccordance with a potential of the gate signal line 925. In the firstperiod, the transistor 961 has a function to supply the video signal tothe pixel 960.

The capacitor 963 has a function to hold the video signal. In the firstperiod, the video signal is supplied to the capacitor 963, which has afunction to hold the video signal. In the second period, the capacitor963 has a function to hold the video signal until the next first period.

As described above, active drive of the pixel 960 can be achieved. Whenthe other transistors over the substrate over which the pixel 960 isformed are n-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

Note that the second electrode of the capacitor 963 can be connected toanywhere as long as the second electrode of the capacitor 963 is held ata constant potential in an operation period of the pixel 960. Forexample, the second electrode of the capacitor 963 may be connected tothe gate signal line 925 of a previous row. This is because the commonline 965 is not required to be provided; therefore, an aperture ratio ofthe pixel 960 is increased.

Note that although a constant potential is supplied to the oppositeelectrode 964, the invention is not limited to this. For example, whenthe pixel 960 is reversely driven, a potential of the opposite electrode964 may be changed corresponding to the reverse drive. At this time, inthe case where the video signal is a positive potential, the potentialof the opposite electrode 964 is a negative potential. In the case wherethe video signal is a negative potential, the potential of the oppositeelectrode 964 is a positive potential.

Although the pixel in FIG. 96 which is formed by using an n-channeltransistor is described, a pixel may be formed by using a p-channeltransistor. Here, FIG. 120 shows a pixel formed by using a p-channeltransistor.

FIG. 120 shows one mode of a pixel. A pixel 1200 in FIG. 120 includes atransistor 1201, the liquid crystal element 962 having the twoelectrodes, and the capacitor 963 having the two electrodes.

As shown in the pixel 1200 of FIG. 120, a first terminal of thetransistor 1201 is connected to the source signal line 924. A secondterminal of the transistor 1201 is connected to the first electrode ofthe liquid crystal element 962 and the first electrode of the capacitor963. A gate terminal of the transistor 1201 is connected to the gatesignal line 925. The second electrode of the liquid crystal element 962is the opposite electrode 964. The second electrode of the capacitor 963is connected to the common line 965.

Note that a video signal is supplied to the source signal line 924. Aselection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that the common potential is supplied to the common line 965. Thesubstrate potential is supplied to the opposite electrode 964. Thecommon potential and the substrate potential are constant potentials.

Note that the liquid crystal element 962, the capacitor 963, theopposite electrode 964, and the common line 965 may be similar to thosein FIG. 96.

The transistor 1201 is a p-channel transistor.

Operations of the pixel 1200 in FIG. 120 in the case where the selectionsignal is supplied to the gate signal line 925 (L level) and the casewhere the selection signal is not supplied (H level) are described,respectively. The first period is a period when the selection signal issupplied to the gate signal line 925. The second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anL level, and the transistor 1201 is turned on. The source signal line924 is electrically connected to the first electrode of the liquidcrystal element 962 and the first electrode of the capacitor 963. Thepotentials of the first electrode of the liquid crystal element 962 andthe first electrode of the capacitor 963 become the same potential asthat of the source signal line 924.

Here, the potential of the source signal line 924 corresponds to thevideo signal.

Light transmittance of the liquid crystal element 962 is determined bythe potential corresponding to the video signal. The potentialcorresponding to the video signal is held in the capacitor 963.

Next, the second period is described. The gate signal line 925 is at anH level, and the transistor 1201 is turned off. The source signal line924 is electrically disconnected from the first electrode of the liquidcrystal element 962 and the first electrode of the capacitor 963.Therefore, the potential corresponding to the video signal inputpreviously is maintained as the potentials of the first electrode of theliquid crystal element 962 and the first electrode of the capacitor 963,and thereby the light transmittance of the liquid crystal element 962 ismaintained as well.

Here, functions of the transistor 1201 and the capacitor 963 aredescribed below.

The transistor 1201 has a function as a switch which selects whether thesource signal line 924 is connected to the first electrode of the liquidcrystal element 962 and the first electrode of the capacitor 963 inaccordance with the potential of the gate signal line 925. In the firstperiod, the transistor 1201 has a function to supply the video signal tothe pixel 1200.

As described above, active drive of the pixel 1200 can be achieved. Whenthe other transistors over the substrate over which the pixel 1200 isformed are p-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

Note that the second electrode of the capacitor 963 can be connected toanywhere as long as the second electrode of the capacitor 963 is held ata constant potential in an operation period of the pixel 1200. Forexample, the second electrode of the capacitor 963 may be connected tothe gate signal line 925 of a previous row. This is because the commonline 965 is not required to be provided; therefore, an aperture ratio ofthe pixel 1200 is increased.

Note that although a constant potential is supplied to the oppositeelectrode 964, the invention is not limited to this. For example, whenthe pixel 1200 is reversely driven, the potential of the oppositeelectrode 964 may be changed corresponding to the reverse drive. At thistime, in the case where the video signal is a positive potential, thepotential of the opposite electrode 964 is a negative potential. In thecase where the video signal is a negative potential, the potential ofthe opposite electrode 964 is a positive potential.

FIG. 97 shows another mode of a pixel. A pixel 970 in FIG. 97 includes atransistor 971, a transistor 972, a display element 973 having twoelectrodes, and a capacitor 974 having two electrodes.

As shown in the pixel 970 of FIG. 97, a first terminal of the transistor971 is connected to the source signal line 924. A second terminal of thetransistor 971 is connected to a gate terminal of the transistor 972 anda first electrode of the capacitor 974. A gate terminal of thetransistor 971 is connected to the gate signal line 925. A secondelectrode of the capacitor 974 is connected to a power supply line 976.A first terminal of the transistor 972 is connected to the power supplyline 976. A second terminal of the transistor 972 is connected to afirst electrode of the display element 973. A second electrode of thedisplay element 973 is a common electrode 975.

Note that a video signal is supplied to the source signal line 924. Aselection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that an anode potential is supplied to the power supply line 976. Acathode potential is supplied to the common electrode 975. The anodepotential is higher than the cathode potential.

Each of the transistors 971 and 972 is an n-channel transistor.

Operations of the pixel 970 in FIG. 97 in the case where the selectionsignal is supplied to the gate signal line 925 (H level) and the casewhere the selection signal is not supplied (L level) are described,respectively. The first period is a period when the selection signal issupplied to the gate signal line 925. The second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anH level, and the transistor 971 is turned on. The source signal line 924is electrically connected to the gate terminal of the transistor 972 andthe first electrode of the capacitor 974. Potentials of the gateterminal of the transistor 972 and the first electrode of the capacitor974 become the same potential as the source signal line 924.

Here, the potential of the source signal line 924 corresponds to thevideo signal.

A current value of the transistor 972 is determined by a potentialdifference (Vgs) between a potential corresponding to the video signaland a potential of the second terminal of the transistor 972, and thesame current as the transistor 972 flows to the display element 973. Inthis case, an operating point of the transistor 972 and the displayelement 973 is required to be set in a saturation region. Thus, acurrent value of the display element 973 can be freely determined by thevideo signal.

Note that when the operating point of the transistor 972 and the displayelement 973 is set in a linear region, the first electrode of thedisplay element 973 is electrically connected to the power supply line976 through the transistor 972, and a voltage approximately equal to apotential of the power supply line 976 is applied to the first electrodeof the display element 973. It is advantageous to set the operatingpoint of the transistor 972 and the display element 973 in the linearregion since the current value of the transistor 972 is not affected bycharacteristics variation and deterioration of the transistor 972.

Next, the case where the selection signal is not supplied to the gatesignal line 925 is described. The gate signal line 925 is at an L level,and the transistor 971 is turned off. The source signal line 924 iselectrically disconnected from the second terminal of the transistor972. Therefore, Vgs of the transistor 972 is held since the potentialcorresponding to the video signal input previously is maintained as thepotential of the second terminal of the transistor 972, and thereby thecurrent value of the display element 973 is held as well.

Here, functions of the transistor 971, the transistor 972, and thecapacitor 974 are described below.

The transistor 971 has a function as a switch which selects whether thesource signal line 924 is connected to the gate terminal of thetransistor 972 and the first electrode of the capacitor 974 inaccordance with a potential of the gate signal line 925. In the firstperiod, the transistor 971 has a function to supply the video signal tothe pixel 970.

The transistor 972 has a function as a driving transistor which suppliesa current or a voltage to the display element 973 in accordance withpotentials of the gate terminal of the transistor 972 and the firstelectrode of the capacitor 974. When the operating point of thetransistor 972 and the display element 973 is set in the saturationregion, the transistor 972 has a function as a current source whichsupplies a current to the display element 973. When the operating pointof the transistor 972 and the display element 973 is set in the linearregion, the transistor 972 has a function as a switch which selectswhether to connect the power supply line 976 and the first electrode ofthe display element 973.

The capacitor 974 has a function to hold the video signal. In the firstperiod, the video signal is supplied to the capacitor 974, which has afunction to hold the video signal. In the second period, the capacitor974 has a function to hold the video signal until the next first period.

As described above, active drive of the pixel 970 can be achieved. Whenthe other transistors over the substrate over which the pixel 970 isformed are n-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

Note that the second electrode of the capacitor 974 can be connected toanywhere as long as the second electrode of the capacitor 974 is held ata constant potential in an operation period of the pixel 970. Forexample, the second electrode of the capacitor 974 may be connected tothe gate signal line 925 of a previous row.

As another example, as shown in a pixel 980 of FIG. 98, the secondelectrode of the capacitor 974 may be connected to the second terminalof the transistor 972. This is because a potential of the gate terminalof the transistor 972 is changed according to change in the potential ofthe second terminal of the transistor 972; therefore, more accuratecurrent is supplied to the display element. That is, when the potentialof the second terminal of the transistor 972 is changed, the potentialof the gate terminal of the transistor 972 is changed simultaneously inaccordance with the capacitive coupling of the capacitor 974. Aso-called bootstrap operation is performed.

Although the pixel in FIG. 97 which is formed by using all n-channeltransistors is described, a pixel may be formed by using all p-channeltransistors. Here, FIG. 121 shows a pixel formed by using all p-channeltransistors.

FIG. 121 shows another mode of a pixel. A pixel 1210 in FIG. 121includes a transistor 1211, a transistor 1212, the display element 973having two electrodes, and the capacitor 974 having two electrodes.

As shown in the pixel 1210 of FIG. 121, a first terminal of thetransistor 1211 is connected to the source signal line 924. A secondterminal of the transistor 1211 is connected to a gate terminal of thetransistor 1212 and the first electrode of the capacitor 974. A gateterminal of the transistor 1211 is connected to the gate signal line925. The second electrode of the capacitor 974 is connected to the powersupply line 976. A first terminal of the transistor 1212 is connected tothe power supply line 976. A second terminal of the transistor 1212 isconnected to the first electrode of the display element 973. The secondelectrode of the display element 973 is the common electrode 975.

Note that a video signal is supplied to the source signal line 924. Aselection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that the anode potential is supplied to the power supply line 976.The cathode potential is supplied to the common electrode 975. The anodepotential is higher than the cathode potential.

Note that the display element 973, the capacitor 974, the commonelectrode 975, and the power supply line 976 may be similar to those inFIG. 97.

The transistor 1211 and the transistor 1212 are p-channel transistors.

Operations of the pixel 1210 in FIG. 121 in the case where the selectionsignal is supplied to the gate signal line 925 (L level) and the casewhere the selection signal is not supplied (H level) are described,respectively. The first period is a period when the selection signal issupplied to the gate signal line 925. The second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anL level, and the transistor 1211 is turned on. The source signal line924 is electrically connected to a gate terminal of the transistor 1212and the first electrode of the capacitor 974. Potentials of the gateterminal of the transistor 1212 and the first electrode of the capacitor974 become the same potential as the source signal line 924.

Here, the potential of the source signal line 924 corresponds to thevideo signal.

A current value of the transistor 1212 is determined by a potentialdifference (Vgs) between a potential corresponding to the video signaland a potential of the power supply line 976, and the same current flowsto the display element 973. In this case, an operating point of thetransistor 1212 and the display element 973 is required to be set in thesaturation region. Thus, a current value of the display element 973 canbe freely determined by the video signal.

Note that when the operating point of the transistor 1212 and thedisplay element 973 is set in a linear region, the first electrode ofthe display element 973 is electrically connected to the power supplyline 976 through the transistor 1212, and a voltage of the firstelectrode of the display element 973 is applied thereto. It isadvantageous to set the operating point of the transistor 1212 and thedisplay element 973 in the linear region since the current value of thetransistor 1212 is not affected by characteristics variation anddeterioration of the transistor 1212.

Next, the case where the selection signal is not supplied to the gatesignal line 925 is described. The gate signal line 925 is at an H level,and the transistor 1211 is turned off. The source signal line 924 iselectrically disconnected from the second terminal of the transistor1212. Therefore, Vgs of the transistor 1212 is held since the potentialcorresponding to the video signal input previously is maintained as apotential of the second terminal of the transistor 1212, and thereby thecurrent value of the display element 973 is held as well.

Here, functions of the transistor 1211 and the transistor 1212 aredescribed below.

The transistor 1211 has a function as a switch which selects whether thesource signal line 924 is connected to the gate terminal of thetransistor 1212 and the first electrode of the capacitor 974 inaccordance with the potential of the gate signal line 925. In the firstperiod, the transistor 1211 has a function to supply the video signal tothe pixel 1210.

The transistor 1212 has a function as a driving transistor whichsupplies a current or a voltage to the display element 973 in accordancewith potentials of the gate terminal of the transistor 1212 and thesecond electrode of the capacitor 974. When the operating point of thetransistor 1212 and the display element 973 is set in the saturationregion, the transistor 1212 has a function as a current source whichsupplies a current to the display element 973. When the operating pointof the transistor 1212 and the display element 973 is set in the linearregion, the transistor 1212 has a function as a switch which selectswhether to connect the power supply line 976 and the first electrode ofthe display element 973.

As described above, active drive of the pixel 970 can be achieved. Whenthe other transistors over the substrate over which the pixel 970 isformed are n-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

Note that the second electrode of the capacitor 974 can be connected toanywhere as long as the second electrode of the capacitor 974 is held ata constant potential in an operation period of the pixel 1210. Forexample, the second electrode of the capacitor 974 may be connected tothe gate signal line 925 of a previous row.

FIG. 99 shows another mode of a pixel. A pixel 990 in FIG. 99 includes atransistor 991, a transistor 992, a transistor 993, the display element973 having two electrodes, and a capacitor 994 having two electrodes.

As shown in the pixel 990 of FIG. 99, a first terminal of the transistor991 is connected to the source signal line 924. A second terminal of thetransistor 991 is connected to a second terminal of the transistor 992,a first electrode of the capacitor 994, and the first electrode of thedisplay element 973. A first terminal of the transistor 992 is connectedto a power supply line 995. A gate terminal of the transistor 992 isconnected to a second terminal of the transistor 993 and a secondelectrode of the capacitor 994. A first terminal of the transistor 993is connected to the gate signal line 925. A gate terminal of thetransistor 993 is connected to the power supply line 995. The secondelectrode of the display element 973 is the common electrode 975.

Note that a video signal is supplied to the source signal line 924. Aselection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that the video signal is an analog current.

Note that a control potential is supplied to the power supply line 995.The cathode potential is supplied to the common electrode. The controlpotential is changed according to operation of the pixel 990.

Note that the display element 973 and the common electrode 975 may besimilar to those in FIG. 97.

The transistors 991, 992 and 993 are n-channel transistors.

Operations of the pixel 990 in FIG. 99 in the case where the selectionsignal is supplied to the gate signal line 925 (H level) and the casewhere the selection signal is not supplied (L level) are described,respectively. The first period is a period when the selection signal issupplied to the gate signal line 925. The second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anH level, and the transistor 991 and the transistor 993 are turned on.The first terminal and the gate terminal of the transistor 992 areelectrically connected through the transistor 993, and the transistor992 is diode-connected. Further, the source signal line 924 iselectrically connected to the second terminal of the transistor 992, thefirst electrode of the capacitor 994, and the first electrode of thedisplay element 973.

At this time, a potential of the power supply line 995 is set so that apotential of the first electrode of the display element 973 is lowerthan a potential of the common electrode 975.

As for the video signal, an analog current which flows from the powersupply line 995 to the source signal line 924 through the transistor 992and the transistor 991 is supplied to the pixel 990. A current same asthe video signal is supplied to the transistor 992. Since the transistor992 is diode-connected, a voltage (Vgs) between the first terminal andthe gate terminal of the transistor 992 at that time is held in thecapacitor 994.

Note that the potential of the first electrode of the display element973 is lower than the potential of the common electrode; therefore, thedisplay element 973 does not emit light.

Next, the second period is described. The gate signal line 925 is at anL level, and the transistor 991 and the transistor 993 are turned off.The first terminal and the gate terminal of the transistor 992 are notelectrically connected through the transistor 993, and the transistor992 is not diode-connected. Further, the source signal line 924 is notelectrically connected to the second terminal of the transistor 992, thefirst electrode of the capacitor 994, and the first electrode of thedisplay element 973.

At this time, a potential of the power supply line 995 is set so thatthe potential of the first electrode of the display element 973 ishigher than the potential of the common electrode 975.

A voltage such that the transistor 992 supplies a current similar to thevideo signal is held in the capacitor 994. When the potential of thepower supply line 995 rises, a potential of the first electrode of thecapacitor 994 also rises. Here, a potential of the gate terminal of thetransistor 992 is raised by the capacitive coupling of the capacitor994, and Vgs of the transistor 992 is held. Therefore, the current sameas the video signal is supplied to the display element 973.

Here, functions of the transistors 991, 992, and 993 and the capacitor994 are described below.

The transistor 991 has a function as a switch which selects whether thesource signal line 924 is connected to the second terminal of thetransistor 992, the first electrode of the capacitor 994, and the firstelectrode of the display element 973 in accordance with the potential ofthe gate signal line 925. In the first period, the transistor 991 has afunction to supply the video signal to the pixel 990.

The transistor 992 has a function as a current source which supplies acurrent to the display element 973 in accordance with potentials of thegate terminal of the transistor 992, the second terminal of thetransistor 993, and the second electrode of the capacitor 994.

The transistor 993 has a function as a switch which selects whether toconnect the first terminal of the transistor 992 and the gate terminalof the transistor 992. In the first period, the transistor 993 has afunction to make the transistor 992 diode-connected.

The capacitor 994 has a function to change the potential of the gateterminal of the transistor 992 in accordance with the potential of thefirst electrode of the display element 973. In the second period, thecapacitor 994 has a function to raise the potential of the gate terminalof the transistor 992 by raising the potential of the first electrode ofthe display element 973.

As described above, active drive of the pixel 990 can be achieved. Whenthe other transistors over the substrate over which the pixel 990 isformed are n-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

FIG. 118 shows another mode of a pixel. A pixel 1180 in FIG. 118includes a transistor 1181, a transistor 1182, a transistor 1183, atransistor 1184, the display element 973 having two electrodes, and thecapacitor 974 having two electrodes.

As shown in the pixel 1180 of FIG. 118, a first terminal of thetransistor 1181 is connected to the source signal line 924. A secondterminal of the transistor 1181 is connected to a second terminal of thetransistor 1182, a gate terminal of the transistor 1183, a gate terminalof the transistor 1184, and the second electrode of the capacitor 974. Agate terminal of the transistor 1181 is connected to the gate signalline 925. A first terminal of the transistor 1182 is connected to afirst terminal of the transistor 1183. A gate terminal of the transistor1182 is connected to the gate signal line 925. A second terminal of thetransistor 1183 is connected to a second terminal of the transistor 1184and the first electrode of the display element 973. A first terminal ofthe transistor 1184 is connected to the power supply line 976. Thesecond electrode of the capacitor 974 is connected to the power supplyline 976. The second electrode of the display element 973 is the commonelectrode 975.

Note that the video signal is supplied to the source signal line 924.The selection signal is supplied to the gate signal line 925. The sourcesignal line 924 and the gate signal line 925 may be similar to those inEmbodiment 1.

Note that the video signal is an analog current.

Note that the anode potential is supplied to the power supply line 976.The cathode potential is supplied to the common electrode 975. The anodepotential is higher than the cathode potential.

Note that the display element 973, the common electrode 975, and thepower supply line 976 may be similar to those in FIG. 97.

The transistors 1181 to 1184 are n-channel transistors.

Operations of the pixel 1180 in FIG. 118 in the case where the selectionsignal is supplied to the gate signal line 925 (H level) and the casewhere the selection signal is not supplied (L level) are described,respectively. The first period is a period when the selection signal issupplied to the gate signal line 925. The second period is a period whenthe selection signal is not supplied.

First, the first period is described. The gate signal line 925 is at anH level, and the transistor 1181 and the transistor 1182 are turned on.The first terminal and the gate terminal of the transistor 1183 areelectrically connected through the transistor 1182, and the transistor1183 is diode-connected. Further, the source signal line 924 iselectrically connected to the first terminal of the transistor 1182, thegate terminal of the transistor 1183, the gate terminal of thetransistor 1184, and the second electrode of the capacitor 974.

As for the video signal, an analog current which flows from the sourcesignal line 924 to the common electrode 975 through the transistor 1181,the transistor 1182, the transistor 1183, and the display element 973 issupplied to the pixel 1180. A current same as the video signal issupplied to the transistor 1183. Since the gate terminal of thetransistor 1183, the gate terminal of the transistor 1184, and thesecond electrode of the capacitor 974 are connected to one another, apotential of the gate terminal of the transistor 1183 at that time isheld in the second electrode of the capacitor 974.

Next, the second period is described. The gate signal line 925 is at anL level, and the transistor 1181 and the transistor 1182 are turned off.The first terminal and the gate terminal of the transistor 1183 are notelectrically connected through the transistor 1182. Further, the sourcesignal line 924 is not electrically connected to the first terminal ofthe transistor 1182, the gate terminal of the transistor 1183, the gateterminal of the transistor 1184, and the second electrode of thecapacitor 974.

The potential corresponding to the video signal is held in the capacitor974. That is, the potential of the gate terminal of the transistor 1183is the same as the potential obtained in the first period. Accordingly,a potential of the gate terminal of the transistor 1184 is the same as apotential of the second electrode of the capacitor 974 as well;therefore, the transistor 1184 can supply a current corresponding to thevideo signal to the display element 973.

Here, functions of the transistors 1181 to 1184 are described below.

The transistor 1181 has a function as a switch which selects whether thesource signal line 924 is connected to the first terminal of thetransistor 1182, the gate terminal of the transistor 1183, the gateterminal of the transistor 1184, and the second electrode of thecapacitor 974 in accordance with the potential of the gate signal line925. In the first period, the transistor 1181 has a function to supplythe video signal to the pixel 1180.

The transistor 1182 has a function as a switch which selects whether toconnect the first terminal of the transistor 1183 and the gate terminalof the transistor 1183 in accordance with the potential of the gatesignal line 925. In the first period, the transistor 1182 has a functionto make the transistor 1183 diode-connected.

The transistor 1183 has a function to determine the potential of thefirst electrode of the display element 973 and the potential of the gateterminal of the transistor 1184 in accordance with the video signal.

The transistor 1184 has a function as a current source which supplies acurrent to the display element 973 in accordance with the potential ofthe second electrode of the capacitor 974.

As described above, active drive of the pixel 1180 can be achieved. Whenthe other transistors over the substrate over which the pixel 1180 isformed are n-channel transistors, simplification of a manufacturingprocess can be realized. Therefore, reduction in manufacturing cost andimprovement in yield can be realized.

Note that the first electrode of the capacitor 974 can be connected toanywhere as long as the first electrode of the capacitor 974 is held ata constant potential in an operation period of the pixel 1180. Forexample, the first electrode of the capacitor 974 may be connected tothe gate signal line 925 of a previous row.

As another example, as shown in a pixel 1190 of FIG. 119, the firstelectrode of the capacitor 974 may be connected to the second terminalof the transistor 1184. This is because the potential of the gateterminal of the transistor 1184 is changed according to change in apotential of the second terminal of the transistor 1184; therefore, moreaccurate current is supplied to the display element. That is, when thesize of the transistor 1183 is different from the size of the transistor1184, a current supplied to the display element 973 is changed;therefore, the potential of the first electrode of the display element973 in the first period and the potential thereof in the second periodare different from each other. Accordingly, the potential of the gateterminal of the transistor 1184 is changed simultaneously in accordancewith the capacitive coupling of the capacitor 974. A so-called bootstrapoperation is performed.

As described above, various pixels can be used for the display device ofthe invention.

Note that in this embodiment, although various pixels are shown, a pixelto which can be applied to the display device of the invention is notlimited to these pixels.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the shift register circuit ofthe invention connected to the pixel described in this embodimentsupplies a power supply potential to the output terminal. Therefore, thepower supply potential is supplied to the output terminal of the shiftregister circuit through the transistor. Since the transistor is notalways on in the non-selection period, the threshold voltage shift ofthe transistor can be suppressed. Further, the power supply potential issupplied to the output terminal of the shift register circuit throughthe transistor at regular intervals. Therefore, the shift registercircuit can suppress noise which is generated in the output terminal.

Embodiment 4

In this embodiment, a structure of a display panel having the pixelstructure shown in the above embodiment is described with reference toFIGS. 100A and 100B.

FIG. 100A is a top plan view showing a display panel and FIG. 100B is across sectional view along A-A′ of FIG. 100A. The display panel includesa signal line control circuit 6701, a pixel portion 6702, a first gatedriver 6703, and a second gate driver 6706, which are shown by dottedlines. The display panel also includes a sealing substrate 6704 and asealing material 6705. A portion surrounded by the sealing material 6705is a space 6707.

Note that a wiring 6708 is for transmitting a signal input to the firstgate driver 6703, the second gate driver 6706, and the signal linecontrol circuit 6701 and receives a video signal, a clock signal, astart signal, and the like from an FPC 6709 (Flexible Printed Circuit)functioning as an external input terminal. An IC chip 6719 (asemiconductor chip including a memory circuit, a buffer circuit, and thelike) is mounted over a connection portion of the FPC 6709 and thedisplay panel by COG (Chip On Glass) or the like. Note that althoughonly the FPC 6709 is shown here, a printed wiring board (PWB) may beattached to the FPC 6709. The display device in this specificationincludes not only a main body of the display panel but also a displaypanel with an FPC or a PWB attached thereto and a display panel on whichan IC chip or the like is mounted.

Next, a cross-sectional structure is described with reference with FIG.100B. The pixel portion 6702 and peripheral driver circuits (the firstgate driver 6703, the second gate driver 6706, and the signal linecontrol circuit 6701) are formed over a substrate 6710. Here, the signalline control circuit 6701 and the pixel portion 6702 are shown.

Note that the signal line control circuit 6701 is formed using a singleconductivity type transistor such as an n-channel transistor 6720 or ann-channel transistor 6721. As for a pixel structure, a pixel can beformed using a single conductivity type transistor by applying the pixelstructure of any of FIGS. 96 to 99, 118 and 119. Accordingly, when theperipheral driver circuits are formed using n-channel transistors, asingle conductivity typesingle conductivity type display panel can bemanufactured. Needless to say, a CMOS circuit may be formed using ap-channel transistor as well as the single conductivity typesingleconductivity type transistor.

Note that in the case where the n-channel transistor 6720 and then-channel transistor 6721 are p-channel transistors, a pixel can beformed using a single conductivity type transistor by applying the pixelstructure of FIG. 120 or 121. Accordingly, when the peripheral drivercircuits are formed using p-channel transistors, a single conductivitytype display panel can be manufactured. Needless to say, a CMOS circuitmay be formed using an n-channel transistor as well as the singleconductivity type transistor.

In this embodiment, although a display panel in which the peripheraldriver circuits are formed over the same substrate as the pixel portionis shown, it is not necessarily required and all or a part of theperipheral driver circuits is formed over an IC chip or the like and theIC chip may be mounted by COG or the like. In that case, the drivercircuit is not required to be single conductivity type and an n-channeltransistor and a p-channel transistor can be used in combination

Further, the pixel portion 6702 includes a transistor 6711 and atransistor 6712. Note that a source electrode of the transistor 6712 isconnected to a first electrode (a pixel electrode 6713). An insulator6714 is formed so as to cover end portions of the pixel electrode 6713.Here, a positive photosensitive acrylic resin film is used for theinsulator 6714.

In order to obtain good coverage, the insulator 6714 is formed to have acurved surface having a curvature at a top end portion or a bottom endportion of the insulator 6714. For example, in the case of using apositive photosensitive acrylic as a material for the insulator 6714, itis preferable that only the top end portion of the insulator 6714 have acurved surface having a curvature radius (0.2 to 3 μm). Further, as theinsulator 6714, either a negative photosensitive acrylic which becomesinsoluble in an etchant by light or a positive photosensitive acrylicwhich becomes soluble in an etchant by light can be used.

A layer 6716 containing an organic compound and a second electrode (anopposite electrode 6717) are formed over the pixel electrode 6713. Here,as a material for the pixel electrode 6713 which functions as an anode,a material having a high work function is preferably used. For example,a single layer of an ITO (indium tin oxide) film, an indium zinc oxide(IZO) film, a titanium nitride film, a chromium film, a tungsten film, aZn film, a Pt film, or the like, a stacked layer of a titanium nitridefilm and a film containing aluminum as a main component, a three-layerstructure of a titanium nitride film, a film containing aluminum as amain component, and a titanium nitride film, or the like can be used.Note that in the case of a stacked layer structure, resistance as awiring is low, good ohmic contact can be obtained, and a function as ananode can be obtained.

The layer 6716 containing an organic compound is formed by anevaporation method using an evaporation mask, or an ink-jet method. Acomplex of a metal belonging to group 4 of the periodic table of theelements is used for a part of the layer 6716 containing an organiccompound, and a low molecular material or a high molecular material maybe used in combination as well. Further, as a material used for thelayer containing an organic compound, a single layer or a stacked layerof an organic compound is often used; however, in this embodiment, aninorganic compound may be used in a part of a film formed of an organiccompound. Moreover, a known triplet material can also be used.

Further, as a material used for the opposite electrode 6717 which isformed over the layer 6716 containing an organic compound, a materialhaving a low work function (Al, Ag, Li, Ca, or an alloy thereof such asMgAg, MgIn, AlLi, calcium fluoride, or calcium nitride) may be used.Note that in the case where light generated from the layer 6716containing an organic compound is transmitted through the oppositeelectrode 6717, a stacked layer of a thin metal film having a thinnerthickness and a transparent conductive film (of ITO (indium tin oxide),indium oxide zinc oxide alloy (In₂O₃—ZnO), zinc oxide (ZnO), or thelike) is preferably used as the opposite electrode 6717 (a cathode).

Further, by attaching the sealing substrate 6704 to the substrate 6710with the sealing material 6705, a light-emitting element 6718 isprovided in the space 6707 surrounded by the substrate 6710, the sealingsubstrate 6704, and the sealing material 6705. Note that the space 6707may be filled with the sealing material 6705 as well as with an inertgas (nitrogen, argon, or the like).

Note that an epoxy-based resin is preferably used for the sealingmaterial 6705. It is preferable that a material for the sealing materialdoes not transmit moisture and oxygen as much as possible. As a materialfor the sealing substrate 6704, a glass substrate, a quartz substrate,or a plastic substrate formed of FRP (Fiberglass-Reinforced Plastics),PVF (polyvinyl fluoride), myler, polyester, acrylic, or the like can beused.

As described above, a display panel having a pixel structure of theinvention can be obtained. Note that the structure described above isonly an example, and a structure of a display panel of the invention isnot limited to this.

As shown in FIGS. 100A and 100B, the signal line control circuit 6701,the pixel portion 6702, the first gate driver 6703, and the second gatedriver 6706 are formed over the same substrate; therefore, reduction incost of the display device can be realized. Further, in this case,single conductivity type transistors are used for the signal linecontrol circuit 6701, the pixel portion 6702, the first gate driver6703, and the second gate driver 6706, thereby simplification of amanufacturing process can be realized; therefore, further cost reductioncan be realized.

Note that the structure of the display panel is not limited to thestructure shown in FIG. 100A where the signal line control circuit 6701,the pixel portion 6702, the first gate driver 6703, and the second gatedriver 6706 are formed over the same substrate, and a signal linecontrol circuit 6801 shown in FIG. 101A corresponding to the signal linecontrol circuit 6701 may be formed over an IC chip and mounted on thedisplay panel by COG or the like. Note that a substrate 6800, a pixelportion 6802, a first gate driver 6803, a second gate driver 6804, anFPC 6805, an IC chip 6806, an IC chip 6807, a sealing substrate 6808,and a sealing material 6809 in FIG. 101A correspond to the substrate6710, the pixel portion 6702, the first gate driver 6703, the secondgate driver 6706, the FPC 6709, the IC chip 6719, the sealing substrate6704, and the sealing material 6705 in FIG. 100A, respectively.

That is, only the signal line control circuit of which high speedoperation is required is formed into an IC chip using a CMOS or thelike, thereby lower power consumption is realized. Further, higher speedoperation and lower power consumption can be achieved by using asemiconductor chip formed of a silicon wafer or the like as the IC chip.

Cost reduction can be realized by forming the first gate driver 6803 andthe second gate driver 6804 over the same substrate as the pixel portion6802. Further, single conductivity type transistors are used for thefirst gate driver 6803, the second gate driver 6804, and the pixelportion 6802; therefore, further cost reduction can be realized. As fora structure of a pixel included in the pixel portion 6802, the pixelsshown in Embodiment 3 can be applied.

As described above, cost reduction of a high-definition display devicecan be realized. Further, by mounting an IC chip including a functionalcircuit (memory or buffer) on a connecting portion of the FPC 6805 andthe substrate 6800, a substrate area can be effectively utilized.

Further, a signal line control circuit 6811, a first gate driver 6814,and a second gate driver 6813 shown in FIG. 101B corresponding to thesignal line control circuit 6701, the first gate driver 6703, and thesecond gate driver 6706 shown in FIG. 100A may be formed over an IC chipand mounted on a display panel by COG or the like. In this case,reduction in power consumption of a high-definition display device canbe realized. Therefore, in order to obtain a display device with lesspower consumption, amorphous silicon is preferably used for asemiconductor layer of a transistor used in the pixel portion. Note thata substrate 6810, a pixel portion 6812, an FPC 6815, an IC chip 6816, anIC chip 6817, a sealing substrate 6818, and a sealing material 6819 inFIG. 101B correspond to the substrate 6710, the pixel portion 6702, theFPC 6709, the IC chip 6719, the IC chip 6719, the sealing substrate6704, and the sealing material 6705 in FIG. 100A, respectively.

In addition, further cost reduction can be realized by using amorphoussilicon for a semiconductor layer of a transistor in the pixel portion6812. Moreover, a large display panel can be manufactured as well.

Further, the second gate driver, the first gate driver, and the signalline control circuit are not required to be provided in a row directionand a column direction of the pixels. For example, a peripheral drivercircuit 6901 formed over an IC chip as shown in FIG. 102A may havefunctions of the first gate driver 6814, the second gate driver 6813,and the signal line control circuit 6811 shown in FIG. 101B. Note that asubstrate 6900, a pixel portion 6902, an FPC 6904, an IC chip 6905, anIC chip 6906, a sealing substrate 6907, and a sealing material 6908 inFIG. 102A correspond to the substrate 6710, the pixel portion 6702, theFPC 6709, the IC chip 6719, the IC chip 6719, the sealing substrate6704, and the sealing material 6705 in FIG. 100A, respectively.

FIG. 102B shows a schematic diagram showing connections of wirings ofthe display device shown in FIG. 102A. The display device includes asubstrate 6910, a peripheral driver circuit 6911, a pixel portion 6912,an FPC 6913, and an FPC 6914. A signal and a power supply potential areexternally input from the FPC 6913 to the peripheral driver circuit6911. An output from the peripheral driver circuit 6911 is input towirings in the row direction and in the column direction, which areconnected to the pixels included in the pixel portion 6912.

FIGS. 103A and 103B show examples of light-emitting elements which canbe applied to the light-emitting element 6718. That is, a structure of alight-emitting element which can be applied to the pixels shown in theabove embodiments is described with reference to FIGS. 103A and 103B.

A light-emitting element in FIG. 103A has an element structure where ananode 7002, a hole injecting layer 7003 formed of a hole injectingmaterial, a hole transporting layer 7004 formed of a hole transportingmaterial, a light emitting layer 7005, an electron transporting layer7006 formed of an electron transporting material, an electron injectinglayer 7007 formed of an electron injecting material, and a cathode 7008are stacked over a substrate 7001. Here, the light emitting layer 7005is formed of only one kind of a light emitting material in some cases,but may also be formed of two or more kinds of materials in other cases.A structure of the element of the invention is not limited to this.

In addition to a stacked layer structure shown in FIG. 103A wherefunctional layers are stacked, there are wide variations such as anelement formed using a high molecular compound, a high efficiencyelement utilizing a triplet light emitting material which emits light inreturning from a triplet excitation state in a light emitting layer.These variations can also be applied to a white light-emitting elementwhich can be obtained by dividing a light emitting region into tworegions by controlling a recombination region of carriers using a holeblocking layer, and the like.

As a manufacturing method of the element of the invention shown in FIG.103A, a hole injecting material, a hole transporting material, and alight emitting material are sequentially deposited over the substrate7001 including the anode 7002 (ITO). Next, an electron transportingmaterial and an electron injecting material are deposited, and finallythe cathode 7008 is formed by evaporation.

Next, materials suitable for the hole injecting material, the holetransporting material, the electron transporting material, the electroninjecting material, and the light emitting material are described asfollows.

As the hole injecting material, an organic compound such as aporphyrin-based compound, phthalocyanine (hereinafter referred to as“H₂Pc”), copper phthalocyanine (hereinafter referred to as “CuPc”), orthe like is available. A material which has a smaller value of anionization potential than that of the hole transporting material to beused and has a hole transporting function can also be used as the holeinjecting material. There are also materials obtained by chemicallydoping a conductive high molecular compound, such as polyaniline,polyethylene dioxythiophene (hereinafter referred to as “PEDOT”) dopedwith polystyrene sulfonate (hereinafter referred to as “PSS”) and thelike. Further, an insulating high molecular compound is effective inplanarization of an anode, and polyimide (hereinafter referred to as“PI”) is often used. Further, an inorganic compound is also used, whichincludes an ultrathin film of aluminum oxide (hereinafter referred to as“alumina”) as well as a thin film of a metal such as gold or platinum.

An aromatic amine-based compound (that is, a compound having a bond ofbenzene ring-nitrogen) is most widely used as the hole transportingmaterial. A material which is widely used as the hole transportingmaterial includes 4,4′-bis(diphenylamino)-biphenyl (hereinafter referredto as “TAD”), derivatives thereof such as4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (hereinafterreferred to as “TPD”), 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl(hereinafter referred to as “α-NPD”), and star burst aromatic aminecompounds such as 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine(hereinafter, referred to as “TDATA”) and4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(hereinafter referred to as “MTDATA”).

As the electron transporting material, a metal complex is often used,which includes a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as Alq, BAlq,tris(4-methyl-8-quinolinolato)aluminum (hereinafter referred to as“Almq”), or bis(10-hydroxybenzo[h]-quinolinato)beryllium (hereinafterreferred to as “BeBq”), and in addition, a metal complex having anoxazole-based or a thiazole-based ligand such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (hereinafter referred to as“Zn(BOX)₂”) or bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (hereinafterreferred to as “Zn(BTZ)₂”). Further, in addition to the metal complexes,oxadiazole derivatives such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (hereinafterreferred to as “PBD”) and OXD-7, triazole derivatives such as TAZ and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-2,3,4-triazole(hereinafter referred to as “p-EtTAZ”), and phenanthroline derivativessuch as bathophenanthroline (hereinafter referred to as “BPhen”) and BCPhave an electron transporting property.

As the electron injecting material, the above-mentioned electrontransporting materials can be used. In addition, an ultrathin film of aninsulator, for example, metal halide such as calcium fluoride, lithiumfluoride, or cesium fluoride, alkali metal oxide such as lithium oxide,or the like is often used. Further, an alkali metal complex such aslithium acetyl acetonate (hereinafter referred to as “Li(acac)”) or8-quinolinolato-lithium (hereinafter referred to as “Liq”) is alsoavailable.

As the light emitting material, in addition to the above-mentioned metalcomplexes such as Alq, Almq, BeBq, BAlq, Zn(BOX)₂, and Zn(BTZ)₂, variousfluorescent pigments are available. The fluorescent pigments include4,4′-bis(2,2-diphenyl-vinyl)-biphenyl, which is blue, and4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, whichis red-orange, and the like. In addition, a triplet light emittingmaterial is available, which mainly includes a complex with platinum oriridium as a central metal. As the triplet light emitting material,tris(2-phenylpyridine)iridium,bis(2-(4′-tolyl)pyridinato-N,C^(2′))acetylacetonato iridium (hereinafterreferred to as “acacIr(tpy)₂”),2,3,7,8,12,13,17,18-octaethyl-21H,23Hporphyrin-platinum, and the likeare known.

By using the materials each having a function as described above incombination, a highly reliable light-emitting element can be formed.

As the display element 973 shown in Embodiment 3, a light-emittingelement in which layers are formed in reverse order of that in FIG. 103Acan be used as shown in FIG. 103B. That is, a cathode 7018, an electroninjecting layer 7017 formed of an electron injecting material, anelectron transporting layer 7016 formed of an electron transportingmaterial, a light emitting layer 7015, a hole transporting layer 7014formed of a hole transporting material, a hole injecting layer 7013formed of a hole injecting material, and an anode 7012 are sequentiallystacked over a substrate 7011.

In addition, at least one of an anode and a cathode of a light-emittingelement is required to be transparent in order to extract lightemission. A transistor and a light-emitting element are formed over asubstrate; and there are light-emitting elements having a top emissionstructure where light emission is extracted from a surface on the sideopposite to the substrate, having a bottom emission structure wherelight emission is extracted from a surface on the substrate side, andhaving a dual emission structure where light emission is extracted fromboth of the surface on the side opposite to the substrate and thesurface on the substrate side. The pixel structure of the invention canbe applied to a light-emitting element having any emission structure.

A light-emitting element having a top emission structure is describedwith reference to FIG. 104A.

A driving TFT 7101 is formed over a substrate 7100. A first electrode7102 is formed in contact with a source electrode of the driving TFT7101, over which a layer 7103 containing an organic compound and asecond electrode 7104 are formed.

The first electrode 7102 is an anode of the light-emitting element. Thesecond electrode 7104 is a cathode of the light-emitting element. Thatis, a region where the layer 7103 containing an organic compound isinterposed between the first electrode 7102 and the second electrode7104 functions as the light-emitting element.

Further, as a material used for the first electrode 7102 which functionsas an anode, a material having a high work function is preferably used.For example, a single layer of a titanium nitride film, a chromium film,a tungsten film, a Zn film, a Pt film, or the like, a stacked layer of atitanium nitride film and a film containing aluminum as a maincomponent, a three-layer structure of a titanium nitride film, a filmcontaining aluminum as a main component, and a titanium nitride film, orthe like can be used. Note that in the case of a stacked layerstructure, the resistance as a wiring is low, a good ohmic contact canbe obtained, and further a function as an anode can be obtained. Byusing a metal film which reflects light, an anode which does nottransmit light can be formed.

As a material used for the second electrode 7104 which functions as acathode, a stacked layer of a thin metal film formed of a materialhaving a low work function (Al, Ag, Li, Ca, or an alloy thereof such asMgAg, MgIn, AlLi, calcium fluoride, or calcium nitride) and atransparent conductive film (of ITO (indium tin oxide), indium zincoxide (IZO), zinc oxide (ZnO), or the like) is preferably used. By usinga thin metal film and a transparent conductive film having a lighttransmitting property, a cathode which can transmit light can be formed.

As described above, light from the light-emitting element can beextracted from the top surface as shown by an arrow in FIG. 104A. Thatis, in the case of applying to the display panel shown in FIGS. 100A and100B, light is emitted to the sealing substrate 6704 side. Therefore, inthe case where a light-emitting element having a top emission structureis applied to a display device, a substrate having a light transmittingproperty is used as the sealing substrate 6704.

In the case of providing an optical film, the sealing substrate 6704 maybe provided with an optical film.

A metal film formed of a material which functions as a cathode and has alow work function, such as MgAg, MgIn, or AlLi can be used for the firstelectrode 7102. For the second electrode 7104, a transparent conductivefilm such as an ITO (indium tin oxide) film or an indium zinc oxide(IZO) film can be used. Therefore, the transmittance of the top lightemission can be improved according to this structure.

Further, a light-emitting element having a bottom emission structure isdescribed with reference to FIG. 104B. The same reference numerals asthose in FIG. 104A are used since the structure of the light-emittingelement is the same except for the light emission structure.

Here, as a material used for the first electrode 7102 which functions asan anode, a material having a high work function is preferably used. Forexample, a transparent conductive film such as an ITO (indium tin oxide)film or an indium zinc oxide (IZO) film can be used. By using atransparent conductive film having a light transmitting property, ananode which can transmit light can be formed.

As a material used for the second electrode 7104 which functions as acathode, a metal film formed of a material having a low work function(Al, Ag, Li, Ca, or an alloy thereof such as MgAg, MgIn, AlLi, calciumfluoride, or Ca₃N₂) can be used. By using a metal film which reflectslight, a cathode which does not transmit light can be formed.

As described above, light from the light-emitting element can beextracted from a bottom surface as shown by an arrow in FIG. 104B. Thatis, in the case of applying to the display panel shown in FIGS. 100A and100B, light is emitted to the substrate 6710 side. Therefore, in thecase where a light-emitting element having a bottom emission structureis applied to a display device, a substrate having a light transmittingproperty is used as the substrate 6710.

In the case of providing an optical film, the substrate 6710 may beprovided with an optical film.

Further, a light-emitting element having a dual emission structure isdescribed with reference to FIG. 104C. The same reference numerals asthose in FIG. 104A are used since the structure of the light-emittingelement is the same except for the light emission structure.

Here, as a material used for the first electrode 7102 which functions asan anode, a material having a high work function is preferably used. Forexample, a transparent conductive film such as an ITO (indium tin oxide)film or an indium zinc oxide (IZO) film can be used. By using atransparent conductive film having a light transmitting property, ananode which can transmit light can be formed.

As a material used for the second electrode 7104 which functions as acathode, a stacked layer of a thin metal film formed of a materialhaving a low work function (Al, Ag, Li, Ca, or an alloy thereof such asMgAg, MgIn, AlLi, calcium fluoride, or calcium nitride) and atransparent conductive film (of ITO (indium tin oxide), indium oxidezinc oxide alloy (In₂O₃—ZnO), zinc oxide (ZnO), or the like) can beused. By using a thin metal film and a transparent conductive filmhaving a light transmitting property, a cathode which can transmit lightcan be formed.

As described above, light from the light-emitting element can beextracted from both sides as shown by arrows in FIG. 104C. That is, inthe case of applying to the display panel shown in FIGS. 100A and 100B,light is emitted to the substrate 6710 side and the sealing substrate6704 side. Therefore, in the case where a light-emitting element havinga dual emission structure is applied to a display device, a substratehaving a light transmitting property is used as each of the substrate6710 and the sealing substrate 6704.

In the case of providing an optical film, each of the substrate 6710 andthe sealing substrate 6704 is provided with an optical film.

In addition, the invention can be applied to a display device whichrealizes full color display by using a white light-emitting element anda color filter.

As shown in FIG. 105, a base film 7202 is formed over a substrate 7200,over which a driving TFT 7201 is formed. A first electrode 7203 isformed in contact with a source electrode of the driving TFT 7201, overwhich a layer 7204 containing an organic compound and a second electrode7205 are formed.

The first electrode 7203 is an anode of a light-emitting element. Thesecond electrode 7205 is a cathode of the light-emitting element. Thatis, a region where the layer 7204 containing an organic compound isinterposed between the first electrode 7203 and the second electrode7205 functions as the light-emitting element. In a structure shown inFIG. 105, white light is emitted. A red color filter 7206R, a greencolor filter 7206G and a blue color filter 7206B are provided over thelight-emitting elements; therefore, full color display can be performed.Further, a black matrix (BM 7207) which separates these color filters isprovided.

The aforementioned structures of the light-emitting element can be usedin combination and can be applied to the display device having the pixelstructure of the invention. The structures of the display panel and thelight-emitting elements which are described above are only examples, andit is needless to say that the pixel structure of the invention can beapplied to display devices having other structures.

Next, a partial cross sectional view of a pixel portion of a displaypanel is described.

First, the case is described where a crystalline semiconductor film(polysilicon (p-Si:H) film) is used as a semiconductor layer of atransistor, with reference to FIGS. 106A, 106B, 107A, and 107B.

The semiconductor layer is obtained by forming an amorphous silicon(a-Si) film over a substrate by a known film formation method, forexample. Note that the semiconductor layer is not limited to theamorphous silicon film, and any semiconductor film having an amorphousstructure (including a microcrystalline semiconductor film) may be used.Further, a compound semiconductor film having an amorphous structure,such as an amorphous silicon germanium film may be used.

Then, the amorphous silicon film is crystallized by a lasercrystallization method, a thermal crystallization method using RTA or anannealing furnace, a thermal crystallization method using a metalelement which promotes crystallization, or the like. Needless to say,such crystallization methods may be performed in combination.

As a result of the aforementioned crystallization, a crystallized regionis formed in a part of the amorphous semiconductor film.

In addition, the crystalline semiconductor film having partiallyincreased crystallinity is patterned into a desired shape, and anisland-shaped semiconductor film is formed using the crystallizedregion. This semiconductor film is used as the semiconductor layer ofthe transistor.

As shown in FIG. 106A, a base film 26102 is formed over a substrate26101, over which a semiconductor layer is formed. The semiconductorlayer includes a channel forming region 26103, an impurity region 26105functioning as a source region or a drain region of the drivingtransistor 26118; and a channel forming region 26106, an LDD region26107, and an impurity region 26108 which function as a lower electrodeof a capacitor 26119. Note that channel doping may be performed to thechannel forming region 26103 and the channel forming region 26106.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. As the basefilm 26102, a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, or a stackedlayer thereof can be used.

A gate electrode 26110 and an upper electrode 26111 of the capacitor areformed over the semiconductor layer with a gate insulating film 26109interposed therebetween.

An interlayer insulator 26112 is formed so as to cover the drivingtransistor 26118 and the capacitor 26119. A wiring 26113 is in contactwith the impurity region 26105 over the interlayer insulator 26112through a contact hole. A pixel electrode 26114 is formed in contactwith the wiring 26113. A second interlayer insulator 26115 is formed soas to cover end portions of the pixel electrode 26114 and the wiring26113. Here, the second interlayer insulator 26115 is formed using apositive photosensitive acrylic resin film. Then, a layer 26116containing an organic compound and an opposite electrode 26117 areformed over the pixel electrode 26114. A light-emitting element 26120 isformed in a region where the layer 26116 containing an organic compoundis interposed between the pixel electrode 26114 and the oppositeelectrode 26117.

In addition, as shown in FIG. 106B, a region 26202 may be provided sothat the LDD region which forms a part of the lower electrode of thecapacitor 26119 is overlapped with the upper electrode 26111. Note thatcommon portions to those in FIG. 106A are denoted by the same referencenumerals, and description thereof is omitted.

In addition, as shown in FIG. 107A, a second upper electrode 26301 maybe provided, which is formed in the same layer as the wiring 26113 incontact with the impurity region 26105 of the driving transistor 26118.Note that common portions to those in FIG. 106A are denoted by the samereference numerals, and description thereof is omitted. A secondcapacitor is formed by interposing the interlayer insulator 26112between the second upper electrode 26301 and the upper electrode 26111.Further, since the second upper electrode 26301 is in contact with theimpurity region 26108, a first capacitor having a structure in which thegate insulating film 26109 is interposed between the upper electrode26111 and the channel forming region 26106; and the second capacitorhaving a structure in which the interlayer insulator 26112 is interposedbetween the upper electrode 26111 and the second upper electrode 26301are connected in parallel, so that a capacitor 26302 having the firstcapacitor and the second capacitor is formed. Since the capacitor 26302has a total capacitance of the first capacitor and the second capacitor,the capacitor having a large capacitance can be formed in a small area.That is, an aperture ratio can be further improved by using thecapacitor in the pixel structure of the invention.

Alternatively, a structure of a capacitor as shown in FIG. 107B may beemployed. A base film 27102 is formed over a substrate 27101, over whicha semiconductor layer is formed. The semiconductor layer includes achannel forming region 27103 and an impurity region 27105 functioning asa source region or a drain region of a driving transistor 27118. Notethat channel doping may be performed to the channel forming region27103.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. As the basefilm 27102, a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, or a stackedlayer thereof can be used.

A gate electrode 27107 and a first electrode 27108 are formed over thesemiconductor layer with a gate insulating film 27106 interposedtherebetween.

A first interlayer insulator 27109 is formed so as to cover the drivingtransistor 27118 and the first electrode 27108. A wiring 27110 is incontact with the impurity region 27105 over the first interlayerinsulator 27109 through a contact hole. In addition, a second electrode27111 is formed in the same layer and with the same material as thewiring 27110.

Further, a second interlayer insulator 27112 is formed so as to coverthe wiring 27110 and the second electrode 27111. A pixel electrode 27113is formed in contact with the wiring 27110 over the second interlayerinsulator 27112 through a contact hole. A third electrode 27114 isformed in the same layer and with the same material as the pixelelectrode 27113. Here, a capacitor 27119 is formed of the firstelectrode 27108, the second electrode 27111, and the third electrode27114.

A third interlayer insulator 27115 is formed so as to cover end portionsof the pixel electrode 27113 and the third electrode 27114. A layer27116 containing an organic compound and an opposite electrode 27117 areformed over the third interlayer insulator 27115 and the third electrode27114. A light-emitting element 27120 is formed in a region where thelayer 27116 containing an organic compound is interposed between thepixel electrode 27113 and the opposite electrode 27117.

As described above, each of the structures shown in FIGS. 106A, 106B,107A, and 107B can be given as an example of a structure of a transistorusing a crystalline semiconductor film for its semiconductor layer. Notethat the transistors having the structures shown in FIGS. 106A, 106B,107A, and 107B are examples of a top gate transistor. That is, thetransistor may be a p-channel transistor or an n-channel transistor. Inthe case of an n-channel transistor, the LDD region may be formed so asto overlap with the gate electrode or not, or a part of the LDD regionmay be formed so as to overlap with the gate electrode. Further, thegate electrode may have a tapered shape and the LDD region may beprovided below the tapered portion of the gate electrode in aself-aligned manner. In addition, the number of gate electrodes is notlimited to two, and a multigate structure with three or more gateelectrodes may be employed, or a single gate structure may also beemployed.

By using a crystalline semiconductor film for a semiconductor layer (achannel forming region, a source region, a drain region, and the like)of a transistor included in the pixel of the invention, for example, thefirst gate driver 6703, the second gate driver 6706, and the signal linecontrol circuit 6701 are easily formed over the same substrate as thepixel portion 6702 in FIGS. 100A and 100B.

As a structure of a transistor which uses polysilicon (p-Si:H) for itssemiconductor layer, each of FIGS. 108A and 108B shows a partial crosssection of a display panel using a transistor having a structure where agate electrode is interposed between a substrate and a semiconductorlayer, that is, a bottom gate structure where a gate electrode islocated below a semiconductor layer.

A base film 7502 is formed over a substrate 7501. A gate electrode 7503is formed over the base film 7502. A first electrode 7504 is formed inthe same layer and with the same material as the gate electrode. As amaterial for the gate electrode 7503, polycrystalline silicon to whichphosphorus is added can be used. Besides polycrystalline silicon,silicide which is a compound of metal and silicon may be used.

Then, a gate insulating film 7505 is formed so as to cover the gateelectrode 7503 and the first electrode 7504. As the gate insulating film7505, a silicon oxide film, a silicon nitride film, or the like is used.

A semiconductor layer is formed over the gate insulating film 7505. Thesemiconductor layer includes a channel forming region 7506, an LDDregion 7507, and an impurity region 7508 functioning as a source regionor a drain region of a driving transistor 7522; and a channel formingregion 7509, an LDD region 7510, and an impurity region 7511, whichfunction as a second electrode of a capacitor 7523. Note that channeldoping may be performed to the channel forming region 7506 and thechannel forming region 7509.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. As the basefilm 7502, a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, or a stackedlayer thereof can be used.

A first interlayer insulator 7512 is formed so as to cover thesemiconductor layer. A wiring 7513 is in contact with the impurityregion 7508 over the first interlayer insulator 7512 through a contacthole. A third electrode 7514 is formed in the same layer and with thesame material as the wiring 7513. The capacitor 7523 is formed of thefirst electrode 7504, the second electrode, and the third electrode7514.

In addition, an opening 7515 is formed in the first interlayer insulator7512. A second interlayer insulator 7516 is formed so as to cover thedriving transistor 7522, the capacitor 7523, and the opening 7515. Apixel electrode 7517 is formed over the second interlayer insulator 7516through a contact hole. Then, an insulator 7518 is formed so as to coverend portions of the pixel electrode 7517. As the insulator, a positivephotosensitive acrylic resin film can be used, for example. A layer 7519containing an organic compound and an opposite electrode 7520 are formedover the pixel electrode 7517. A light-emitting element 7521 is formedin a region where the layer 7519 containing an organic compound isinterposed between the pixel electrode 7517 and the opposite electrode7520. The opening 7515 is located below the light-emitting element 7521.That is, when light emitted from the light-emitting element 7521 isextracted from the substrate side, the transmittance can be improvedsince the opening 7515 is provided.

Further, a structure shown in FIG. 108B in which a fourth electrode 7524is formed in the same layer and with the same material as the pixelelectrode 7517 in FIG. 108A may be employed. Therefore, the capacitor7523 can be formed of the first electrode 7504, the second electrode,the third electrode 7514, and the fourth electrode 7524.

Next, the case where an amorphous silicon (a-Si:H) film is used for asemiconductor layer of a transistor is described. FIGS. 109A and 109Bshow the case of a top gate transistor. FIGS. 110A, 110B, 111A, and 111Bshow the case of a bottom gate transistor.

FIG. 109A shows a cross section of a transistor having a forwardstaggered structure, which uses amorphous silicon for its semiconductorlayer. A base film 7602 is formed over a substrate 7601. A pixelelectrode 7603 is formed over the base film 7602. A first electrode 7604is formed in the same layer and with the same material as the pixelelectrode 7603.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. As the basefilm 7602, a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, or a stackedlayer thereof can be used.

A wiring 7605 and a wiring 7606 are formed over the base film 7602, andan end portion of the pixel electrode 7603 is covered with the wiring7605. An n-type semiconductor layer 7607 and an n-type semiconductorlayer 7608 which have an n-type conductivity are formed over the wiring7605 and the wiring 7606, respectively. In addition, a semiconductorlayer 7609 is formed between the wiring 7605 and the wiring 7606 andover the base film 7602. A part of the semiconductor layer 7609 isextended over the n-type semiconductor layer 7607 and the n-typesemiconductor layer 7608. Note that this semiconductor layer is formedof a non-crystalline semiconductor film such as an amorphous silicon(a-Si:H) film or a microcrystalline semiconductor (μ-Si:H) film.Further, a gate insulating film 7610 is formed over the semiconductorlayer 7609. An insulating film 7611 is formed in the same layer and withthe same material as the gate insulating film 7610 and also formed overthe first electrode 7604. Note that as the gate insulating film 7610, asilicon oxide film, a silicon nitride film, or the like is used.

A gate electrode 7612 is formed over the gate insulating film 7610. Asecond electrode 7613 which is formed in the same layer and with thesame material as the gate electrode is formed over the first electrode7604 with the insulating film 7611 interposed therebetween. A capacitor7619 in which the insulating film 7611 is interposed between the firstelectrode 7604 and the second electrode 7613 is formed. An interlayerinsulator 7614 is formed so as to cover an end portion of the pixelelectrode 7603, the driving transistor 7618, and the capacitor 7619.

A layer 7615 containing an organic compound and an opposite electrode7616 are formed over the interlayer insulator 7614 and the pixelelectrode 7603 located in an opening of the interlayer insulator 7614. Alight-emitting element 7617 is formed in a region where the layer 7615containing an organic compound is interposed between the pixel electrode7603 and the opposite electrode 7616.

A first electrode 7620 as shown in FIG. 109B may be formed instead ofthe first electrode 7604 shown in FIG. 109A. The first electrode 7620 isformed in the same layer and with the same material as the wirings 7605and 7606.

FIGS. 110A and 110B are partial cross sections of a display panelincluding a bottom gate transistor which uses amorphous silicon for itssemiconductor layer.

A base film 7702 is formed over a substrate 7701. A gate electrode 7703is formed over the base film 7702. A first electrode 7704 is formed inthe same layer and with the same material as the gate electrode 7703. Asa material for the gate electrode 7703, polycrystalline silicon to whichphosphorus is added can be used. Besides polycrystalline silicon,silicide which is a compound of metal and silicon may be used.

Then, a gate insulating film 7705 is formed so as to cover the gateelectrode 7703 and the first electrode 7704. As the gate insulating film7705, a silicon oxide film, a silicon nitride film, or the like is used.

A semiconductor layer 7706 is formed over the gate insulating film 7705.In addition, a semiconductor layer 7707 is formed in the same layer andwith the same material as the semiconductor layer 7706.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. As the basefilm 7602, a single layer of aluminum nitride (AlN), silicon oxide(SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like or a stackedlayer thereof can be used.

N-type semiconductor layers 7708 and 7709 having n-type conductivity areformed over the semiconductor layer 7706. An n-type semiconductor layer7710 is formed over the semiconductor layer 7707.

Wirings 7711 and 7712 are formed over the n-type semiconductor layers7708 and 7709, respectively. A conductive layer 7713 formed in the samelayer and with the same material as the wirings 7711 and 7712, areformed over the n-type semiconductor layer 7710.

A second electrode is formed with the semiconductor layer 7707, then-type semiconductor layer 7710, and the conductive layer 7713. Notethat a capacitor 7720 having a structure where the gate insulating film7705 is interposed between the second electrode and the first electrode7704 is formed.

One end portion of the wiring 7711 is extended, and a pixel electrode7714 is formed so as to be in contact with an upper potion of theextended wiring 7711.

An insulator 7715 is formed so as to cover an end portion of the pixelelectrode 7714, a driving transistor 7719, and the capacitor 7720.

A layer 7716 containing an organic compound and an opposite electrode7717 are formed over the pixel electrode 7714 and the insulator 7715. Alight-emitting element 7718 is formed in a region where the layer 7716containing an organic compound is interposed between the pixel electrode7714 and the opposite electrode 7717.

The semiconductor layer 7707 and the n-type semiconductor layer 7710which are a part of the second electrode of the capacitor are notnecessarily required to be formed. That is, the second electrode may bethe conductive layer 7713, so that the capacitor may have a structure inwhich the gate insulating film is interposed between the first electrode7704 and the conductive layer 7713.

Note that in FIG. 110A, the pixel electrode 7714 may be formed beforeforming the wiring 7711, thereby the capacitor 7720 as shown in FIG.110B can be formed, which has a structure where the gate insulating film7705 is interposed between the first electrode 7704 and a secondelectrode 7721 formed of the pixel electrode 7714.

Note that although FIGS. 110A and 110B show inverted staggeredchannel-etched transistors, a channel protective transistor may also beused. A channel protective transistor is described with reference toFIGS. 111A and 111B.

A channel protective transistor shown in FIG. 111A is different from thedriving transistor 7719 having a channel-etched structure shown in FIG.110A in that an insulator 7801 functioning as an etching mask isprovided over a region where a channel of the semiconductor layer 7706is to be formed. Common portions except that point are denoted by thesame reference numerals.

Similarly, a channel protective transistor shown in FIG. 111B isdifferent from the driving transistor 7719 having a channel-etchedstructure shown in FIG. 110B in that an insulator 7802 functioning as anetching mask is provided over the region where a channel of thesemiconductor layer 7706 is to be formed. Common portions except thatpoint are denoted by the same reference numerals.

By using an amorphous semiconductor film as a semiconductor layer (achannel forming region, a source region, a drain region, and the like)of a transistor included in the pixel of the invention, themanufacturing cost can be reduced. For example, an amorphoussemiconductor film can be applied by using the pixel structure shown inEmbodiment 3.

Note that structures of the transistors and the capacitor to which thepixel structure of the invention can be applied are not limited to thosedescribed above, and transistors and capacitor with various structurescan be used.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the shift register circuit ofthe invention connected to the display panel described in thisembodiment supplies a power supply potential to the output terminal.Therefore, the power supply potential is supplied to the output terminalof the shift register circuit through the transistor. Since thetransistor is not always on in the non-selection period, the thresholdvoltage shift of the transistor can be suppressed. Further, the powersupply potential is supplied to the output terminal of the shiftregister circuit through the transistor at regular intervals. Therefore,the shift register circuit can suppress noise which is generated in theoutput terminal.

Embodiment 5

The display device of the invention can be applied to various electronicdevices, specifically to display portions of electronic devices. Theelectronic devices include cameras such as a video camera and a digitalcamera, a goggle-type display, a navigation system, an audio reproducingdevice (a car audio component stereo, an audio component stereo, or thelike), a computer, a game machine, a portable information terminal (amobile computer, a mobile phone, a mobile game machine, an electronicbook, or the like), an image reproducing device provided with arecording medium (specifically, a device for reproducing content of arecording medium such as a digital versatile disc (DVD) and having adisplay for displaying the reproduced image) and the like.

FIG. 117A shows a display, which includes a housing 84101, a supportingbase 84102, a display portion 84103, and the like. A display devicehaving the pixel structure of the invention can be used for the displayportion 84103. Note that the display includes all display devices fordisplaying information such as for a personal computer, TV broadcastingreception, and advertisement display. A display using the display devicehaving the pixel structure of the invention for the display portion84103 can reduce power consumption and prevent a display defect.Further, cost reduction can be achieved.

In recent years, the need for a large size display has been increased.As a display becomes larger, there is caused a problem of increasedcost. Therefore, it is an issue to reduce the manufacturing cost as muchas possible and to provide a high quality product at as low a price aspossible.

For example, by applying the pixel structure shown in Embodiment 3 to apixel portion of a display panel, a display panel formed by using singleconductivity type transistors can be provided. Therefore, the number ofmanufacturing steps can be reduced and the manufacturing cost can bereduced.

In addition, by forming the pixel portion and the peripheral drivercircuit over the same substrate as shown in FIG. 100A, the display panelcan be formed using circuits constituted by single conductivity typetransistors.

In addition, by using an amorphous semiconductor (such as amorphoussilicon (a-Si:H)) for a semiconductor layer of a transistor in a circuitincluded in the pixel portion, a manufacturing process can be simplifiedand further cost reduction can be realized. In this case, as shown inFIGS. 101B and 102A, it is preferable that the periphery driver circuitin the pixel portion be formed over an IC chip and mounted on thedisplay panel by COG or the like. In this manner, by using an amorphoussemiconductor, the size of the display can be easily increased.

FIG. 117B shows a camera, which includes a main body 84201, a displayportion 84202, an image receiving portion 84203, operation keys 84204,an external connection port 84205, a shutter 84206, and the like.

In recent years, in accordance with advance in performance of a digitalcamera and the like, competitive manufacturing thereof has beenintensified. Thus, it is important to provide a higher-performanceproduct at as low a price as possible. A digital camera using a displaydevice having the pixel structure of the invention for the displayportion 84202 can reduce power consumption and prevent a display defect.Further, cost reduction can be achieved.

For example, by using the pixel structure shown in Embodiment 3 for thepixel portion, the pixel portion can be constituted by singleconductivity type transistors. In addition, as shown in FIG. 101A, asignal line control circuit of which operating speed is high is formedover an IC chip, and a gate driver of which operating speed isrelatively low with a circuit constituted by single conductivity typetransistors over the same substrate as the pixel portion; therefore,higher performance can be realized and cost reduction can be achieved.Further, an amorphous semiconductor such as amorphous silicon may beused for the pixel portion and a semiconductor layer of a transistorincluded in the gate driver which is formed over the same substrate asthe pixel portion; therefore, further cost reduction can be achieved.

FIG. 117C shows a computer, which includes a main body 84301, a housing84302, a display portion 84303, a keyboard 84304, an external connectionport 84305, a pointing device 84306, and the like. A computer using adisplay device having the pixel structure of the invention for thedisplay portion 84303 can reduce power consumption and prevent a displaydefect. Further, cost reduction can be achieved.

FIG. 117D shows a mobile computer, which includes a main body 84401, adisplay portion 84402, a switch 84403, operation keys 84404, an infraredport 84405, and the like. A mobile computer using a display devicehaving the pixel structure of the invention for the display portion84402 can reduce power consumption and prevent a display defect.Further, cost reduction can be achieved.

FIG. 117E shows a portable image reproducing device having a recordingmedium (specifically, a DVD player), which includes a main body 84501, ahousing 84502, a display portion A 84503, a display portion B 84504, arecording medium reading portion 84505, operation keys 84506, a speakerportion 84507, and the like. The display portion A 84503 mainly displaysimage information and the display portion B 84504 mainly displays textinformation. An image reproducing device using a display device havingthe pixel structure of the invention for the display portion A 84503 andthe display portion B 84504 can reduce power consumption and prevent adisplay defect. Further, cost reduction can be achieved.

FIG. 117F shows a goggle-type display, which includes a main body 84601,a display portion 84602, an earphone 84603, and a support portion 84604.A goggle type display using a display device having the pixel structureof the invention for the display portion 84602 can reduce powerconsumption and prevent a display defect. Further, cost reduction can beachieved.

FIG. 117G shows a mobile game machine, which includes a housing 84701, adisplay portion 84702, a speaker portion 84703, operation keys 84704, arecording medium insert portion 84705, and the like. A portable typegame machine using a display device having the pixel structure of theinvention for the display portion 84702 can reduce power consumption andprevent a display defect. Further, cost reduction can be achieved.

FIG. 117H shows a digital camera having a television receiving function,which includes a main body 84801, a display portion 84802, operationkeys 84803, a speaker 84804, a shutter 84805, an image receiving portion84806, an antenna 84807, and the like. A digital camera having atelevision receiving function using a display device having the pixelstructure of the invention for the display portion 84802 can reducepower consumption and prevent a display defect. In addition,high-definition display with a high aperture ratio can be achieved.Further, cost reduction can be achieved.

For example, the pixel structures of FIGS. 96 to 99, 118 and 119 areused in the pixel portion; therefore, an aperture ratio of a pixel canbe increased. Specifically, the aperture ratio can be increased by usingan n-channel transistor for a driving transistor for driving alight-emitting element. Thus, a digital camera having a televisionreceiving function which includes a high-definition display portion canbe provided.

While a digital camera having a television receiving function becomesmultifunctional and frequency of use thereof, such as televisionwatching, has been increased, the battery life per charge has beenrequired to be long.

For example, as shown in FIGS. 101B and 102A, a peripheral drivercircuit is formed over an IC chip and a CMOS or the like is used;therefore, power consumption can be reduced.

As described above, the invention can be applied to various electronicdevices.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the shift register circuit ofthe invention connected to the electronic device described in thisembodiment supplies a power supply potential to the output terminal.Therefore, the power supply potential is supplied to the output terminalof the shift register circuit through the transistor. Since thetransistor is not always on in the non-selection period, the thresholdvoltage shift of the transistor can be suppressed. Further, the powersupply potential is supplied to the output terminal of the shiftregister circuit through the transistor at regular intervals. Therefore,the shift register circuit can suppress noise which is generated in theoutput terminal.

Embodiment 6

In this embodiment, a structure example of a mobile phone which includesa display portion having a display device using the pixel structure ofthe invention is described with reference to FIG. 116.

A display panel 8301 is detachably incorporated in a housing 8330. Theshape and the size of the housing 8330 can be changed as appropriate inaccordance with the size of the display panel 8301. The housing 8330which fixes the display panel 8301 is fitted in a printed circuit board8331 so as to be assembled as a module.

The display panel 8301 is connected to the printed circuit board 8331through an FPC 8313. A speaker 8332, a microphone 8333, atransmitting/receiving circuit 8334, and a signal processing circuit8335 including a CPU, a controller, and the like are formed over theprinted circuit board 8331. Such a module, an input unit 8336, a battery8337 and an antenna 8340 are combined and stored in a housing 8339. Apixel portion of the display panel 8301 is provided so as to be seenfrom an opening window formed in the housing 8339.

In the display panel 8301, a pixel portion and a part of peripheraldriver circuits (a driver circuit with a low operation frequency among aplurality of driver circuits) may be formed over the same substrateusing transistors, a part of the peripheral driver circuits (a drivercircuit with a high operation frequency among the plurality of drivercircuits) may be formed over an IC chip, and the IC chip may be mountedon the display panel 8301 by COG (Chip On Glass). Alternatively, the ICchip may be connected to a glass substrate by using TAB (Tape AutomatedBonding) or a printed circuit board. According to such a structure,power consumption of a display device can be reduced and the batterylife of a mobile phone per charge can be made long. In addition, costreduction of the mobile phone can be achieved.

As the pixel portion, the pixel structures shown in the aboveembodiments can be applied as appropriate.

For example, by applying the pixel structure shown in Embodiment 3 orthe like, the number of manufacturing steps can be reduced. That is, thepixel portion and the peripheral driver circuit formed over the samesubstrate as the pixel portion are constituted by single conductivitytype transistors; therefore, cost reduction can be achieved.

In addition, in order to further reduce power consumption, the pixelportion may be formed over a substrate by using transistors, all of theperipheral driver circuits may be formed over an IC chip, and the ICchip may be mounted on the display panel by COG (Chip On Glass) or thelike as shown in FIGS. 101B and 102A.

Note that the structure shown in this embodiment is only an example of amobile phone, and the pixel structure of the invention can be appliednot only to a mobile phone having the aforementioned structure but alsoto mobile phones having various structures.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the shift register circuit ofthe invention included in the mobile phone described in this embodimentsupplies a power supply potential to the output terminal. Therefore, thepower supply potential is supplied to the output terminal of the shiftregister circuit through the transistor. Since the transistor is notalways on hi the non-selection period, the threshold voltage shift ofthe transistor can be suppressed. Further, the power supply potential issupplied to the output terminal of the shift register circuit throughthe transistor at regular intervals. Therefore, the shift registercircuit can suppress noise which is generated in the output terminal.

Embodiment 7

In this embodiment, a structure example of an electronic device whichincludes a display portion having a display device using the pixelstructure of the invention, and in particular, a television receiverincluding an EL module is described.

FIG. 112 shows an EL module combining a display panel 7901 and a circuitboard 7911. The display panel 7901 includes a pixel portion 7902, a scanline driver circuit 7903, and a signal line driver circuit 7904. Acontrol circuit 7912, a signal dividing circuit 7913, and the like areformed over the circuit board 7911. The display panel 7901 and thecircuit board 7911 are connected to each other by a connection wiring7914. As the connection wiring, an FPC or the like can be used.

In the display panel 7901, the pixel portion 7902 and a part ofperipheral driver circuits (a driver circuit with a low operationfrequency among a plurality of driver circuits) may be formed over thesame substrate using transistors, a part of the peripheral drivercircuits (a driver circuit with a high operation frequency among theplurality of driver circuits) may be formed over an IC chip, and the ICchip may be mounted on the display panel 7901 by COG (Chip On Glass) orthe like. Alternatively, the IC chip may be mounted on the display panel7901 by using TAB (Tape Automated Bonding) or a printed circuit board.

As the pixel portion, the pixel structure shown in the above embodimentscan be applied as appropriate.

For example, by applying the pixel structure and the like shown inEmbodiment 3, the number of manufacturing steps can be reduced. That is,the pixel portion and the peripheral driver circuit formed over the samesubstrate as the pixel portion are constituted by single conductivitytype transistors; therefore, cost reduction can be achieved.

In addition, in order to further reduce power consumption, the pixelportion may be formed over a glass substrate by using transistors, allof the peripheral driver circuits may be formed into an IC chip, and theIC chip may be mounted on the display panel by COG (Chip On Glass) orthe like.

In addition, pixels can be constituted only by n-channel transistors byapplying the pixel structures shown in FIGS. 96 to 99, 118 and 119 ofthe above embodiments; therefore, an amorphous semiconductor (such asamorphous silicon) can be applied to a semiconductor layer of atransistor. That is, a large display device where it is difficult toform an even crystalline semiconductor film can be manufactured.Further, by using an amorphous semiconductor film for a semiconductorlayer of a transistor constituting a pixel, the number of manufacturingsteps can be reduced and manufacturing cost can also be reduced.

Note that in the case where an amorphous semiconductor film is appliedto a semiconductor layer of a transistor constituting a pixel, it ispreferable that the pixel portion be formed over a substrate by usingtransistors, all of the peripheral driver circuits be formed over an ICchip, and the IC chip be mounted on the display panel by COG (Chip OnGlass). FIG. 101B shows an example of the structure where a pixelportion is formed over a substrate and an IC chip provided with aperipheral driver circuit is mounted on the substrate by COG or thelike.

An EL television receiver can be completed with this EL module. FIG. 113is a block diagram showing a main structure of an EL televisionreceiver. A tuner 8001 receives a video signal and an audio signal. Thevideo signals are processed by a video signal amplifier circuit 8002, avideo signal processing circuit 8003 which converts a signal output fromthe video signal amplifier circuit 8002 into a color signalcorresponding to each color of red, green and blue, and a controlcircuit 8012 which converts the video signal into the inputspecification of a driver circuit. The control circuit 8012 outputs asignal to each of a scan line side and a signal line side. Whenperforming digital drive, a structure where a signal dividing circuit8013 is provided on the signal line side in order that an input digitalsignal is divided into m signals to be supplied.

Among the signals received by the tuner 8001, an audio signal istransmitted to an audio signal amplifier circuit 8004, and an outputthereof is supplied to a speaker 8007 through an audio signal processingcircuit 8005. A control circuit 8008 receives control data on receivingstation (receiving frequency) and volume from an input portion 8009 andtransmits signals to the tuner 8001 and the audio signal processingcircuit 8005.

FIG. 114A shows a television receiver incorporating an EL module havinga different mode from that in FIG. 113. In FIG. 114A, a display screen8102 is constituted by the EL module. In addition, a speaker 8103,operation switches 8104, and the like are provided in a housing 8101 asappropriate.

FIG. 114B shows a television receiver having a portable wirelessdisplay. A battery and a signal receiver are incorporated into a housing8112. A display portion 8113 and a speaker portion 8117 are driven bythe battery. The battery can be repeatedly charged by a battery charger8110. The battery charger 8110 can transmit and receive a video signaland transmit the video signal to the signal receiver of the display. Thehousing 8112 is controlled by operation keys 8116. The device shown inFIG. 114B can also be referred to as a video-audio bidirectionalcommunication device since a signal can be sent from the housing 8112 tothe battery charger 8110 by operating the operation keys 8116. Thedevice can also be referred to as a versatile remote control devicesince a signal can be sent from the housing 8112 to the battery charger8110 by operating the operation keys 8116 and another electronic deviceis made to receive a signal which can be sent by the battery charger8110, and accordingly, communication control of another electronicdevice is realized. The invention can be applied to the display portion8113.

FIG. 115A shows a module formed by combining a display panel 8201 and aprinted wiring board 8202. The display panel 8201 includes a pixelportion 8203 provided with a plurality of pixels, a first gate driver8204, a second gate driver 8205, and a signal line driver circuit 8206which supplies a video signal to a selected pixel.

The printed wiring board 8202 is provided with a controller 8207, acentral processing unit (CPU 8208), a memory 8209, a power supplycircuit 8210, an audio processing circuit 8211, a transmitting/receivingcircuit 8212, and the like. The printed wiring board 8202 is connectedto the display panel 8201 thorough a flexible printed circuit 8213(FPC). The printed wiring board 8202 can be formed to have a structurein which a capacitor, a buffer circuit, and the like are provided inorder to prevent noise on a power supply voltage or a signal, or dullsignal rising. The controller 8207, the audio processing circuit 8211,the memory 8209, the CPU 8208, the power supply circuit 8210, and thelike can be mounted to the display panel 8201 by using a COG (Chip OnGlass) method. By using a COG method, the size of the printed wiringboard 8202 can be reduced.

Various control signals are input and output through an interfaceportion (I/F portion 8214) which is included in the printed wiring board8202. An antenna port 8215 for transmitting and receiving a signalto/from an antenna is included in the printed wiring board 8202.

FIG. 115B is a block diagram of the module shown in FIG. 115A. Themodule includes a VRAM 8216, a DRAM 8217, a flash memory 8218, and thelike as a memory 8209. The VRAM 8216 stores data on an image displayedon a panel, the DRAM 8217 stores video data or audio data, and the flashmemory stores various programs.

The power supply circuit 8210 supplies electric power for operating thedisplay panel 8201, the controller 8207, the CPU 8208, the audioprocessing circuit 8211, the memory 8209, and the transmitting/receivingcircuit 8212. Depending on a panel specification, the power supplycircuit 8210 is provided with a current source in some cases.

The CPU 8208 includes a control signal generation circuit 8220, adecoder 8221, a register 8222, an arithmetic circuit 8223, a RAM 8224,an interface 8219 for the CPU 8208, and the like. Various signals inputto the CPU 8208 via the interface 8219 are once stored in the register8222, and subsequently input to the arithmetic circuit 8223, the decoder8221, or the like. The arithmetic circuit 8223 performs operation basedon the input signal so as to designate a location to which variousinstructions are sent. On the other hand, the signal input to thedecoder 8221 is decoded and input to the control signal generationcircuit 8220. The control signal generation circuit 8220 generates asignal including various instructions based on the input signal, andtransmits the signal to the designated location by the arithmeticcircuit 8223, specifically the location such as the memory 8209, thetransmitting/receiving circuit 8212, the audio processing circuit 8211,and the controller 8207.

The memory 8209, the transmitting/receiving circuit 8212, the audioprocessing circuit 8211, and the controller 8207 are operated inaccordance with the instructions received thereby respectively.Hereinafter, the operation is briefly described.

The signal input from an input unit 8225 is sent to the CPU 8208 mountedto the printed wiring board 8202 via the I/F portion 8214. The controlsignal generation circuit 8220 converts video data stored in the VRAM8216 into a predetermined format depending on the signal sent from theinput unit 8225 such as a pointing device or a keyboard, and transmitsthe converted data to the controller 8207.

The controller 8207 performs data processing of the signal including thevideo data sent from the CPU 8208 in accordance with the panelspecification and supplies the signal to the display panel 8201.Further, the controller 8207 generates an Hsync signal, a Vsync signal,a clock signal CLK, an alternating voltage (AC Cont), and a switchingsignal L/R based on a power supply voltage from the power supply circuit8210 or various signals input from the CPU 8208 and supplies the signalsto the display panel 8201.

A signal which is to be received and sent by an antenna 8228 as anelectric wave is processed by the transmitting/receiving circuit 8212.Specifically, the transmitting/receiving circuit 8212 includes ahigh-frequency circuit such as isolator, a band pass filter, a VCO(Voltage Controlled Oscillator), an LPF (Low Pass Filter), a coupler, ora balun. A signal including audio information among signals transmittedand received in the transmitting/receiving circuit 8212 is sent to theaudio processing circuit 8211 in accordance with an instruction from theCPU 8208.

The signal including audio information which is sent in accordance withthe instruction from the CPU 8208 is demodulated into an audio signal inthe audio processing circuit 8211 and is sent to a speaker 8227. Anaudio signal sent from a microphone 8226 is modulated in the audioprocessing circuit 8211 and is sent to the transmitting/receivingcircuit 8212 in accordance with an instruction from the CPU 8208.

The controller 8207, the CPU 8208, the power supply circuit 8210, theaudio processing circuit 8211, and the memory 8209 can be mounted as apackage according to this embodiment.

Needless to say, the invention is not limited to the televisionreceiver. The invention can be applied to various usages especially as alarge display medium such as an information display board at a railwaystation or an airport, an advertisement display board on the street, orthe like, in addition to a monitor of a personal computer.

Note that this embodiment can be freely implemented in combination withany description in other embodiment modes and embodiments in thisspecification. That is, in a non-selection period, the transistor isturned on at regular intervals, so that the shift register circuit ofthe invention included in the electronic device described in thisembodiment supplies a power supply potential to the output terminal.Therefore, the power supply potential is supplied to the output terminalof the shift register circuit through the transistor. Since thetransistor is not always on in the non-selection period, the thresholdvoltage shift of the transistor can be suppressed. Further, the powersupply potential is supplied to the output terminal of the shiftregister circuit through the transistor at regular intervals. Therefore,the shift register circuit can suppress noise which is generated in theoutput terminal.

This application is based on Japanese Patent Application serial No.2006-001941 filed in Japan Patent Office on Jan. 7, 2006, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: a firsttransistor comprising a gate terminal, a first terminal, and a secondterminal; a second transistor comprising a gate terminal, a firstterminal, and a second terminal; a third transistor comprising a gateterminal, a first terminal, and a second terminal; a fourth transistorcomprising a gate terminal, a first terminal, and a second terminal; afifth transistor comprising a gate terminal, a first terminal, and asecond terminal; a sixth transistor comprising a gate terminal, a firstterminal, and a second terminal; a seventh transistor comprising a gateterminal, a first terminal, and a second terminal; and an eighthtransistor comprising a gate terminal, a first terminal, and a secondterminal; wherein the first terminal of the first transistor iselectrically connected to the gate terminal of the second transistor,the first terminal of the fourth transistor, the gate terminal of thefifth transistor, and the first terminal of the seventh transistor,wherein the first terminal of the second transistor is electricallyconnected to the first terminal of the third transistor and an outputterminal, wherein the gate terminal of the third transistor iselectrically connected to the gate terminal of the fourth transistor,and the first terminal of the eighth transistor, wherein the firstterminal of the fifth transistor is electrically connected to the firstterminal of the sixth transistor, and the gate terminal of the eighthtransistor, wherein the gate terminal of the sixth transistor iselectrically connected to the second terminal of the sixth transistor,and wherein the second terminal of the eighth transistor is electricallyconnected to a wiring configured to be supplied with a control signal.2. The semiconductor device according to claim 1, wherein the secondterminal of the first transistor is electrically connected to a firstpower supply.
 3. The semiconductor device according to claim 1, whereinthe gate terminal of the sixth transistor and the second terminal of thesixth transistor is electrically connected to a first power supply. 4.The semiconductor device according to claim 1, wherein the secondterminal of the eighth transistor is electrically connected to an inputterminal.
 5. The semiconductor device according to claim 1, wherein thesecond terminal of the second transistor is electrically connected to aninput terminal.
 6. The semiconductor device according to claim 1,wherein the second terminal of the seventh transistor is electricallyconnected to a second power supply.
 7. The semiconductor deviceaccording to claim 1, wherein the output terminal is electricallyconnected to a buffer circuit.
 8. A semiconductor device comprising: afirst transistor comprising a gate terminal, a first terminal, and asecond terminal; a second transistor comprising a gate terminal, a firstterminal, and a second terminal; a third transistor comprising a gateterminal, a first terminal, and a second terminal; a fourth transistorcomprising a gate terminal, a first terminal, and a second terminal; afifth transistor comprising a gate terminal, a first terminal, and asecond terminal; a sixth transistor comprising a gate terminal, a firstterminal, and a second terminal; a seventh transistor comprising a gateterminal, a first terminal, and a second terminal; an eighth transistorcomprising a gate terminal, a first terminal, and a second terminal; anda capacitor comprising a first electrode and a second electrode, whereinthe first terminal of the first transistor is electrically connected tothe gate terminal of the second transistor, the first terminal of thefourth transistor, the gate terminal of the fifth transistor, the firstterminal of the seventh transistor, and the first electrode of thecapacitor, wherein the first terminal of the second transistor iselectrically connected to the first terminal of the third transistor thesecond electrode of the capacitor, and an output terminal, wherein thegate terminal of the third transistor is electrically connected to thegate terminal of the fourth transistor, and the first terminal of theeighth transistor, wherein the first terminal of the fifth transistor iselectrically connected to the first terminal of the sixth transistor,and the gate terminal of the eighth transistor, wherein the gateterminal of the sixth transistor is electrically connected to the secondterminal of the sixth transistor, and wherein the second terminal of theeighth transistor is electrically connected to a wiring configured to besupplied with a control signal.
 9. The semiconductor device according toclaim 8, wherein the second terminal of the first transistor iselectrically connected to a first power supply.
 10. The semiconductordevice according to claim 8, wherein the gate terminal of the sixthtransistor and the second terminal of the sixth transistor iselectrically connected to a first power supply.
 11. The semiconductordevice according to claim 8, wherein the second terminal of the eighthtransistor is electrically connected to an input terminal.
 12. Thesemiconductor device according to claim 8, wherein the second terminalof the second transistor is electrically connected to an input terminal.13. The semiconductor device according to claim 8, wherein the secondterminal of the seventh transistor is electrically connected to a secondpower supply.
 14. The semiconductor device according to claim 8, whereinthe output terminal is electrically connected to a buffer circuit.
 15. Asemiconductor device comprising: a first flip-flop circuit, a secondflip-flop circuit, and a third flip-flop circuit each comprising: afirst transistor comprising a gate terminal, a first terminal, and asecond terminal; a second transistor comprising a gate terminal, a firstterminal, and a second terminal; a third transistor comprising a gateterminal, a first terminal, and a second terminal; a fourth transistorcomprising a gate terminal, a first terminal, and a second terminal; afifth transistor comprising a gate terminal, a first terminal, and asecond terminal; a sixth transistor comprising a gate terminal, a firstterminal, and a second terminal; a seventh transistor comprising a gateterminal, a first terminal, and a second terminal; an eighth transistorcomprising a gate terminal, a first terminal, and a second terminal; anda capacitor comprising a first electrode and a second electrode, whereinthe first terminal of the first transistor is electrically connected tothe gate terminal of the second transistor, the first terminal of thefourth transistor, the gate terminal of the fifth transistor, the firstterminal of the seventh transistor, and the first electrode of thecapacitor, wherein the first terminal of the second transistor iselectrically connected to the first terminal of the third transistor,the second electrode of the capacitor, and an output terminal, whereinthe gate terminal of the third transistor is electrically connected tothe gate terminal of the fourth transistor, and the first terminal ofthe eighth transistor, wherein the first terminal of the fifthtransistor is electrically connected to the first terminal of the sixthtransistor, and the gate terminal of the eighth transistor, and whereinthe gate terminal of the sixth transistor is electrically connected tothe second terminal of the sixth transistor; wherein the gate of thefirst transistor in the second flip-flop circuit is electricallyconnected to the output terminal of the first flip-flop circuit, whereinthe gate of the seventh transistor in the second flip-flop circuit iselectrically connected to the output terminal of the third flip-flopcircuit, wherein the second terminal of the eighth transistor in thefirst flip-flop circuit is electrically connected to a first wiringconfigured to be supplied with a first control signal, and wherein thesecond terminal of the eighth transistor in the second flip-flop circuitis electrically connected to a second wiring configured to be suppliedwith a second control signal.
 16. The semiconductor device according toclaim 15, wherein the second terminal of the first transistor iselectrically connected to a first power supply.
 17. The semiconductordevice according to claim 15, wherein the gate terminal of the sixthtransistor and the second terminal of the sixth transistor iselectrically connected to a first power supply.
 18. The semiconductordevice according to claim 15, wherein the second terminal of the eighthtransistor is electrically connected to an input terminal.
 19. Thesemiconductor device according to claim 15, wherein the second terminalof the second transistor is electrically connected to an input terminal.20. The semiconductor device according to claim 15, wherein the secondterminal of the seventh transistor is electrically connected to a secondpower supply.
 21. The semiconductor device according to claim 15,wherein the output terminal is electrically connected to a buffercircuit.